AN ABSTRACT OF THE THESIS OF
Barbara A. Lagerquist for the degree of Masters of Science in Wildlife Science
presented on May 13. 1997. Title: Dive Characteristics of Northeast Pacific Blue
Whales (Balaenoptera musculus) Monitored by Satellite Telemetry.
Redacted for Privacy
Abstract approved:
Bruce R. Mate
The dive habits of Northeast Pacific blue whales (Balaenoptera musculus) were
studied using satellite-monitored radio tags. Four blue whales were tagged and tracked
off the central California coast, near the Gulf of the Farallones, in 1993, two off the
southern California coast, near the Channel Islands, in 1994, and six more near the
Channel Islands in 1995.
The 1993 tags summarized dive duration data into eight 3-h periods daily. One tag
additionally summarized dive depth and time at depth information for these same
periods. Tracking periods ranged from 0.6-12.7 days, and provided data for 17, 3-h
summary periods, representing 2,007 dives (788 of which provided depth
information). Seventy-one percent of dives were <1 min (n = 2,007). Data suggests
that total number of dives during summary periods differed among animals (range of
>Vs = 83-128, p = 0.07). Average duration of true dives (> 1 min) also differed
among animals (range of R's = 4.2-7.2 min, p = 0.04). All whales spent >94 % of
their time submerged. Seventy-five percent of dives of the depth-monitored whale
were <16 m, and 78 % of this animal's time was spent in the upper 16 m of the
water column. Average depth of true dives (>16 m) was
105
m, with the maximum
depth being 192 m. Depth and duration of the deepest dive were not correlated.
Although there were no significant diel differences in dive duration variables,
trends were apparent. Two whales made frequent (R' = 91, SE = 10, n = 4; x =
121, n = 1), short-duration ()V = 6.7, SE =
0.9,
n = 4; R
= 5.0
min, n = 1) dives
at night. Depth of sounding dives for one of these whales was shallower at night (3F =
98, SE =
4
m, n = 6) than during the day (5? = 120, SE = 1 m, n = 3, p <0 .01) .
A third whale dove more frequently and for shorter duration during the day (x- = 87,
SE = 17 dives, R = 7.1, SE = 0.9 min, n = 2) than at night (rc = 79, SE = 8
dives, . = 7.4, SE = 1.6 min, n = 2). The fourth whale dove more frequently, but
for longer duration during the day (r( =
min, n =
4),
141,
SE = 22 dives, 5? =
4.5,
SE = 1.2
than at night (5e = 102, SE = 23 dives, R = 3.8, SE = 0.3 min, n =
2).
The
1994-95
tags collected duration and depth information on every dive/surface
event, and stored this information as both individual dive/surface data and summary
data for eight 3-h periods daily. Tracking periods ranged from 0.9-39.4 days, and
provided data from 203, 3-h summary periods, and 1,822 individual dive/surface
events. Depths and durations of dives were correlated for all whales (r = 0.55, p
<0.001). Surface intervals and dive durations were correlated for five whales (p' s
<0.05).
Surface intervals for three of these whales were more closely related to the
previous dive than to the subsequent dive. All dive variables differed among whales
(p <0.001). After controlling for individual differences, diel differences were found
for maximum surface duration and maximum dive duration during a summary period.
Significant diel differences in surface durations, dive durations, and dive depths, were
also found for a number of whales individually. Dive variables also differed
depending on water depth in which the whales were located. Significant diel
differences were exhibited when whales were over slope waters (200-2,000 m), but
not while over the continental shelf (0-200 m), or the ocean floor (2,000+ m).
Movement information obtained for one whale permitted classification into non-
directed and directed modes of travel. Dive characteristics differed significantly
between the two modes of travel (p' s <0.001).
Individual dive/surface data was also collected visually for comparison with tag
data, and differences are discussed. Dive durations and surface intervals were
positively correlated (r = 0.50, p <0.001), with surface intervals being more closely
related to the duration of the previous dive (r = 0.54, p <0.001) than to the duration
of the subsequent dive (r = 0.40, p <0.001). Differences existed in many of the
visually-observed dive variables depending on whale behavior.
°Copyright by Barbara A. Lagerquist
May 13, 1997
All Rights Reserved
Dive Characteristics of Northeast Pacific Blue Whales
(Balaenoptera musculus) Monitored by Satellite Telemetry
by
Barbara A. Lagerquist
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
Completed May 13, 1997
Commencement June 1998
Master of Science thesis of Barbara A. Lagerquist presented on May 13. 1997
APPROVED:
Redacted for Privacy
7111F41"rrofessor, representing Wildlife Science
Redacted for Privacy
Chair of Department o 0:"" eries
d Wildlife
Redacted for Privacy
Dean of Graduat
chool
I understand that my thesis will become part of the permanent collection of Oregon
State University libraries. My signature below authorizes release of my thesis to any
reader upon request.
Redacted for Privacy
Barbara A.
g quist, uthor
CONTRIBUTION OF AUTHORS
Dr. Bruce R. Mate was involved in the design and implementation of the research
projects outlined in both chapters, and edited the manuscripts. Kathleen M. Stafford
assisted in data collection and was involved in the analysis and writing of depth of
dive sections for chapter one.
TABLE OF CONTENTS
Page
INTRODUCTION
1
CHAPTER ONE
Satellite-Monitored Dive Characteristics of Blue Whales (Balaenoptera
musculus) off the Central California Coast
6
INTRODUCTION
7
METHODS
9
Depth Tag
Duration Tag
Visual Observations
Tag Data
Statistical Analysis
RESULTS
Percentage of Time Submerged
Total Number of Dives
Average Dive Duration
Average Duration of Dives > 1 min
Maximum Dive Duration
Maximum Surface Duration
Depth
Time at Depth
Visual Observational Data
Movements
DISCUSSION
Movements
Dive Duration
Dive Depth
Tag Data vs Visual Data
Comparative Discussion
10
11
12
14
16
16
17
17
17
21
21
22
22
24
24
26
28
28
30
32
34
35
TABLE OF CONTENTS (Continued)
Page
CHAPTER TWO
Satellite-Monitored Dive Characteristics of Blue Whales (Balaenoptera
musculus) off the Southern California Coast
38
INTRODUCTION
39
METHODS
41
Visual Observations
Tag Data
Statistical Analysis
RESULTS
Visual Observations
Pooled Data Analysis
Analysis of Means
Tag Results
Diel comparisons
Travel mode comparisons
Bathymetrical comparisons
DISCUSSION
43
45
46
47
47
47
50
51
56
64
67
71
SUMMARY
79
BIBLIOGRAPHY
82
LIST OF FIGURES
Figure
Page
Average number of dives in each of eight duration categories, during 17
summary periods, for all blue whales tagged off central California, 1993
20
Average percentage of dives to each of six depth categories (n = 9) and
average percentage of time spent in each of six depth categories (n = 7)
during 3-h summary periods for whale 10836, tagged off central
California, 1993
23
1.3
Tagging and satellite-derived locations for whales 10836 and 834
27
2.1
Log-survivorship plot of all times between surfacings for 25 untagged
blue whales observed off southern California, 1995
48
Log-survivorship plot of all times between surfacings for eight blue
whales tagged off southern California, 1994/95
55
1.1
1.2
2.2
LIST OF TABLES
Table
1.1
Page
Means (± SE's) of dive duration data, by time of day, for all blue
whales tagged off central California, 1993
18
1.2
Comparison of visually-observed surface/respiration activities for several
species of Mysticete whales
25
2.1
Means ± SE's of visual observation variables, by behavior category, for
untagged blue whales off southern California, 1995
51
Deployment dates, number of days for which messages were received,
and number of messages received for the eight blue whales tagged off
southern California, 1994/95
52
Means ± SE's of all dive variables for all blue whales tagged off
southern California, 1994/95
53
Means ± SE's of summary period variables, by time of day, for all blue
whales tagged off southern California, 1994/95
58
Means ± SE's of individual dive variables, by time of day, for all blue
whales tagged off southern California, 1994/95
60
Means ± SE's of visual observation-style variables, by time of day, for
all blue whales tagged off southern California, 1994/95
62
Means ± SE's of summary period variables, by travel mode, for blue
whale 841, tagged off southern California, 1995
65
Means ± SE's of individual dive variables, by travel mode, for blue
whale 841, tagged off southern California, 1995
65
Means ± SE's of visual observation-style variables, by travel mode, for
blue whale 841, tagged off southern California, 1995
66
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10 Means ± SE's of summary period variables, by bottom-depth category,
for all blue whales tagged off southern California, 1994/95
68
LIST OF TABLES (Continued)
Page
Table
2.11
2.12
Means ± SE's of individual dive variables, by bottom-depth category,
for all blue whales tagged off southern California, 1994/95
68
Means ± SE's of visual observation-style variables, by bottom-depth
category, for all blue. whales tagged off southern California, 1994/95 ...
69
Dive Characteristics of Northeast Pacific Blue Whales
(Balaenoptera musculus) Monitored by Satellite Telemetry
INTRODUCTION
The blue whale (Balaenoptera musculus) is the largest animal ever to live on Earth
(Leatherwood and Reeves 1983), with average lengths at physical maturity of 24.1 m
and 25.0 m for males and females, respectively (Braham 1984, Mizroch et al. 1984,
Mansfield 1985). They are found in all oceans (Mizroch et al. 1984, Mansfield 1985),
primarily along the edge of continental shelves and along ice fronts, but also
venturing into deep oceanic zones and shallow inshore regions (Leatherwood and
Reeves 1983). Blue whales are a migratory species, traveling from summer feeding
grounds in polar and temperate waters, to warmer, subtropical breeding and calving
grounds during the winter (Leatherwood and Reeves 1983). They are almost
completely stenophagous, feeding primarily on euphausiids (Leatherwood and Reeves
1983, Rice 1986). The species most commonly consumed in the North Pacific include
Euphausia pacifica, Thysanoessa spp., and Nematoscelis megalops (Gaskin 1982,
Rice 1986).
Compared to many whales, blue whales are relatively fast swimmers, traveling at
5-33 km/h when cruising, and capable of swimming 20-48 km/h when chased (Slijper
1962, Lockyer 1981). They were, therefore, inaccessible to early commercial whalers
in their row boats (Brueggeman et al. 1985). It was not until the invention of the
steam-engined ship and exploding harpoon in Norway in 1864, that blue whales
2
became targets for commercial whaling operations (Mizroch et al. 1984, Yochem and
Leatherwood 1985).
Because of their immense size, blue whales were the most sought after species by
commercial whalers (Braham 1984, Mansfield 1985, Yochem and Leatherwood
1985). Whaling began in the eastern North Pacific in 1905, and took place along the
California, Oregon, Washington, British Columbia, and Alaska coastlines (Mizroch et
al. 1984). Peak catches occurred in 1926, when 239 blue whales were taken. As a
result of over-exploitation, blue whale populations were severely depleted, leading to
a world-wide ban on their commercial harvest in 1966 (Horwood 1986). It is believed
that there were approximately 4,900 blue whales in the North Pacific before whaling
began (Leatherwood and Reeves 1983, Braham 1984). No reliable estimates were
available for the population size immediately following the commercial ban, but ten
years later they were estimated to be at 33% of the initial population size (1,600
animals; Mizroch et al. 1984).
The blue whales found off California coastal waters are thought to make up one of
several populations of blue whales in the North Pacific (Barlow et al. 1995). While
initial abundance estimates of this population do not exist, these animals were almost
certainly depleted by whaling (Barlow et al. 1995), and they continue to be listed as
endangered under the Endangered Species Act. The current estimate for this
population is 2,134 (CV =0.27; Barlow et al. 1995).
3
Increasing numbers of blue whales have been seen in the Gulf of the Farallones,
California, beginning in the late 1970's (Calambokidis et al. 1990) as well as in the
Channel Islands, California, in recent years (Barlow 1995). The size of this increase
is too large to be accounted for by population growth alone, and more likely
represents a shift in distribution to more coastal waters (Barlow 1995). Increases in
blue whale sightings have not been reported in other areas of the North Pacific
(Calambokidis et al. 1990).
Despite the cessation of commercial whaling, blue whales have not been immune
to human-caused mortality and injury. Ship strikes have been implicated in blue whale
deaths in four of the last 16 years (Barlow et al. 1995). Offshore drift gillnet fisheries
have also been identified as potential threats to blue whales in California and Mexico
(Barlow et al. 1995). Physical, chemical, or acoustic pollution, may also directly, or
indirectly affect blue whale survival. Physical pollution, or marine debris, could be a
source of injury and mortality through entanglement in lost and discarded netting or
fishing line, or ingestion of objects that may block or puncture digestive tracts
(Marine Mammal Commission 1996). Chemical contaminants in the ocean may have
either acute lethal effects, or sub-lethal effects, in the form of immune system
suppression (Marine Mammal Commission 1996). Acoustic pollution, depending on
the nature and intensity of the sound, may injure or kill blue whales, cause temporary
or permanent hearing loss, or cause whales to abandon important feeding, breeding,
4
or resting areas, or migratory routes, which could indirectly affect their survival
(Marine Mammal Commission 1996).
Management and conservation of blue whales cannot be effective without accurate
information regarding the abundance, distribution, and critical habitat requirements of
this species. Only the most general information on these topics has been provided in
the past, primarily through visual observations of whales from land, ships, or
airplanes. Such techniques are fraught with limitations, not the least of which is the
fact that blue whales are often too far offshore to be observed from land, and ship and
aerial time is labor intensive and costly. Visual observations are also limited to
favorable weather conditions, daylight hours, and times when whales are at the
ocean's surface.
The remote tracking of whales through the use of satellite-monitored radio tags
avoids many of the limitations associated with visual observations. Satellite-telemetry
allows for the monitoring of many whales simultaneously, anywhere in the world.
Whales can be tracked in this way at all times of the day, and in all weather
conditions, without the need for expensive follow-up labor and equipment. In
addition, information regarding whales' behavior underwater, as well as at the ocean
surface, can be obtained.
The objectives of the following studies were to determine the feasibility of using
satellite telemetry to remotely track free-ranging blue whales, with the intention of
providing more detailed information regarding their movements, and their use of the
5
ocean environment. Tags capable of recording and transmitting information on the
durations and depths of dives, as well as duration of surfacings, were deployed on
blue whales off the central and southern coasts of California, in the summer/fall of
1993, 1994, and 1995. The information received allowed for characterization of blue
whale dive habits, and the examination of relationships between such dive
characteristics and time of day, or geographical area. Comparisons of dive
characteristics both within and among species were also examined.
The information gained in these studies is not only valuable in providing baseline
data on a species for which little is known, but also in providing surfacing rate
information useful in the development of more accurate abundance estimation.
6
CHAPTER 1
Dive Characteristics of Satellite-Monitored Blue Whales
(Balaenoptera musculus) off the Central California coast
Barbara A. Lagerquist, Kathleen M. Stafford,
and Bruce R. Mate
Department of Fisheries and Wildlife
Oregon State University
Corvallis, Oregon
7
INTRODUCTION
Few studies were conducted on blue whales (Balaenoptera musculus) in the North
Pacific in the 20 years following the 1966 ban on commercial whaling of this species
(Calambokidis et al. 1990). As such, information on the seasonal distribution,
abundance, and population characteristics of blue whales was incomplete (Rice and
Wolman 1982, Brueggeman et al. 1985). In the last 10 years studies have addressed
these issues (Calambokidis et al. 1990, Reilly and Thayer 1990, Tershy et al. 1990,
Wade and Gerrodette 1993, Barlow 1994 and 1995). However, with the exception of
Schoenherr's (1991) observation of feeding blue whales in Monterey Canyon,
behavioral studies are still lacking.
Respiration rates and diving patterns of blue whales are poorly documented
(Leatherwood et al. 1982). Respiration patterns are critical in transforming ship-board
and aerial survey counts into accurate abundance estimates (Doi 1974, CETAP 1982,
Hiby and Hammond 1989). Diel variations in activity, which might render whales
more visible at certain times of day, are also significant factors to be taken into
account in assessment techniques (Klinowska 1986). Several studies have addressed
these subjects for other species (Wursig et al. 1984, Wursig et al. 1986, Dolphin
1987, Dorsey et al. 1989, Joyce et al. 1990, Stern 1992, Stone et al. 1992, Kopelman
and Sadove 1995), but none have considered the blue whale.
Prey distribution positively influences behavior for a number of cetacean species
(Evans 1971, Volkov and Moroz 1977, Shane et al. 1986, Smith et al. 1986, Wursig
8
and Bastida 1986, Dolphin 1987, Goodyear 1989). It has been suggested that blue and
other baleen whales feed most frequently in the morning and evening (Nishiwaki and
Oye 1951, Nemoto 1959, Marr 1962). Blue whales are almost completely
stenophagous (Gaskin 1982, Rice 1986), feeding primarily on euphausids (Rice 1963,
Nemoto 1970, Nemoto and Kawamura 1977). The species most commonly consumed
in the north Pacific include Euphausia pacifica, Nematoscelis megalops, Thysanoessa
longipes, T. inermis, and T. spinifera (Zenkovich 1936, Pike 1950, Nemoto 1959,
Nemoto and Kasuya 1965, Tomilin 1967, Kawamura 1980, Rice 1986). Blue whales
are thought to be relatively shallow feeders, as most krill are distributed within 100 m
of the surface during part if not all of the day (Laurie 1933, Brinton 1962, Marr
1962, Huntley et al. 1995). However, many species of krill exhibit diel variations in
their vertical movements, migrating to shallower depths during darkness, and to
deeper water (100 500 meters) during daylight (Brinton 1962, Mauch line and Fisher
1969, Gaskin 1982, Croxall et al. 1985). Thus, feeding at night would allow blue
whales to take advantage of shallower, more accessible prey. Schoenherr (1991),
however, observed blue whales feeding on surface swarms of T. spinifera during
daytime in Monterey Bay, California. Daytime surface swarms of T. spinifera are
common along the California coast during spring and summer, when upwelling events
are most frequent and intense (Smith and Adams 1988). During such swarming events
blue whales might be expected to feed both day and night.
9
Recognizing and monitoring the movements and dive patterns of whales is
difficult because they spend the majority of their time underwater (Leatherwood and
Evans 1979, Dolphin 1987). Conventional VHF and HF radio telemetry studies have
made significant advances toward understanding whale movements and dive patterns,
despite expensive logistics (Evans et al. 1971, Norris et al. 1974, Watkins 1978,
Leatherwood and Evans 1979, Harvey and Mate 1984, Mate and Harvey 1984, Frost
et al. 1985). Further improvements have been made with satellite telemetry, which
can track a number of whales simultaneously, without expensive ships and associated
labor for short-distance monitoring (Tanaka 1987, Tanaka et al. 1988, Mate 1989,
Martin and Smith 1992, Mate et al. 1994 and 1995, Martin et al. 1994).
In this study, we monitored the movements and dive habits of blue whales in the
eastern North Pacific via satellite telemetry. The goals of the study were to test the
feasibility of this technique to remotely monitor dive behaviors in a large, freeranging cetacean and to document diel rhythms in diving behavior.
METHODS
Four blue whales were tagged with satellite-monitored radio transmitters off the
coast of central California from 20 Aug - 30 Sept, 1993. Tags were deployed 1-4 m
behind the blowhole by use of a 68-kg crossbow, from a 5.3-m, rigid-hulled inflatable
boat. Following tagging, animals were photographed with 35 mm cameras with 80­
200-mm lenses, for purposes of identification, and documentation of tag attachment.
10
The UHF transmitters emitted 400-mW signals to Argos receivers on two NOAA
TIROS-N weather satellites in sun-synchronous polar orbits. The transmitters were
programmed to transmit during selected daily periods coinciding with the 10-12 times
during which satellites were overhead for approximately 6-17 min. This strategy
conserved battery power. During each transmission cycle, the tags transmitted every
40 sec when at the surface. A saltwater conductivity switch was used to assure the tag
was at the surface before initiating a transmission. The tags were powered by organic
lithium batteries.
Two types of sensor tags were used: a "duration" type, which collected dive
duration information only, and a "depth" type, which collected information
concerning dive depth as well as duration. We defined a dive as any submergence
greater than 6 sec. Tag locations were calculated by Service Argos from Doppler shift
data when two or more messages reached the satellite during one pass (Argos 1990).
Depth Tag
One depth tag was deployed on a blue whale. This tag consisted of a Telonics
(Mesa, AZ) ST-6 Argos transmitter, a Wildlife Computers (Woodinville, WA)
controller board, and a pressure transducer. Transmissions were scheduled for four 2­
h periods daily. A 960-ms transmission consisted of a discrete identification code and
256 bits of sensor data, which included a cyclic redundancy check code (CRC) for
error detection. Data on every dive were collected and summarized over eight 3-h
11
summary periods daily. The summary information included the number of dives in ten
different depth categories, ranging from 0-2040 m; the number of dives in each of
eight duration categories, ranging from 6 sec 255 min; the percent of time spent in
each of nine depth categories, ranging from 0-2040 m; the duration of the longest
dive (2-min resolution), duration of deepest dive (2-min resolution), depth of deepest
dive (16-m resolution), temperature at deepest depth (2-°C resolution), longest surface
duration uninterrupted by a submergence of greater than 6 sec (30-sec resolution), and
total surface duration (1-min resolution).
The tag housing was a stainless steel cylinder 5 cm in diameter by 19 cm in length
and weighed 0.80 kg with two sub-dermal attachments. Attachments consisted of
stainless steel rods (12.7 cm long, 6 mm in diameter) with double-edged blades at the
distal end. One pair of folding toggles was mounted behind the blades to prevent
outward migration of the tag. The tag was filled with plastic epoxy to reduce air
spaces and add structural strength. A flexible 17-cm whip antenna was mounted in
one end-cap perpendicular to the tag housing.
Duration Tag
Three duration tags were deployed on blue whales. Each consisted of an Oregon
State University-designed sampling program loaded in a Telonics ST-6 Argos
transmitter (Telonics, Mesa, AZ). The housings and attachments of these tags were
12
identical to the depth tags with the exception that they lacked a pressure transducer.
Duration tags were not filled with plastic epoxy and weighed 0.52 kg.
Duration tags collected data on every dive, and summarized the information over
eight 3-h summary periods daily. The data collected during each period consisted of
the number of dives occurring in each of eight duration categories, ranging from 6 sec
- 255 min; maximum surface duration (1-min resolution); maximum dive duration
greater than 10 min (2-min resolution); and percentage of time submerged (1-percent
resolution).
Transmissions were scheduled for two 100-min periods each day. During each
320-ms transmission, 64 bits of data and a CRC code were sent to the satellites.
Visual Observations
Whale respiration patterns were observed from a 16.6-m vessel (R/V 'Cille) using
the focal animal sampling technique (Altmann 1974). Before beginning an
observation, we took general behavioral observations and identification photographs
of the whales. If more than one whale was present, the most easily distinguishable
animal was chosen for respiration sampling. When it was possible to distinguish both
animals in a pair by distinctive coloration or dorsal fin shape, respiration data were
collected for both. Observation distances varied from 10-150 m.
The desired sampling period for each bout was 30 min. Bouts were terminated
prior to 30 min in cases where the focal animal could not be identified, either due to
13
poor weather conditions or losing track of the focal whale. The primary observer
called out sightings to a second person who recorded exact times (h:min:sec) of
exhalations and behaviors
.
Subsequently, four variables similar to those measured in
other respiration studies (Wursig et al. 1984, Wursig et al. 1986, Dolphin 1987,
Dorsey et al. 1989) were evaluated: dive time (of dives greater than 1 min); duration
of time at the surface between successive dives (surface time); number of blows
during the surfacing; and the time between blows (blow interval). As in Dolphin's
(1987) study, duration of time at the surface was given the value of 0.1 min when the
surface time included only 1 blow. Mean blow interval for each bout was calculated
by dividing the sum of all blow intervals in a surfacing by the number of blow
intervals, and then taking the average of that value. Mean blow rate was calculated as
described by Dorsey et al. (1989), by dividing mean number of blows per surfacing
by mean surfacing-dive cycle (mean duration of surfacing plus mean duration of the
following dive). Surface time proportion was calculated by dividing mean surface
time by mean surfacing-dive cycle.
Krill samples were collected with a swimming pool net (mesh size 1.5 X 2.0
mm) by sweeping the net through surface swarms. Species were identified by C.
Miller, Oregon State University.
14
Tag Data
All messages containing CRC errors were eliminated from data analysis. The
average duration of all dives during a summary period was calculated using the
following formula:
xduri = ( ptsub; * 1.8)/tdivesi
where xduri = the average dive duration during period i; ptsub; = the percent time
submerged during period i; and tdivesi = the total number of dives during period i.
The multiple 1.8 represents 180 min during a 3-h summary period divided by 100,
and is used to convert ptsub from a percentage to measure of time (min).
The average duration of dives greater than one min in length was calculated using
the following formula:
8
xdur60i = E mid value;;
* dives;; / tdivesi
)=2
where xdur60; = average duration of dives greater than 1 min for period i; mid value;;
= middle value (min) of duration category j in period i, excluding the first category
(dives < 60 sec); dives;; = number of dives in duration category j for period i,
excluding the first category.
15
The average depth of all dives during a summary period was calculated using the
following formula:
10
xdepi = E mid-depth4 * dives4 / tdivesi
)=1
where xdepi = average dive depth for period i; mid depth;; = middle value (m) of
depth category j in period i; dives;; = number of dives in depth category j for period
i.
Because dives were defined as submergences greater than 6 sec, the depth tag did
not begin counting dive duration until 6 sec had elapsed. The first 6 sec of every dive
was not included in the duration. For periods with an exact count of dives, the
number of dives was multiplied by 6 sec. This amount was added to the time spent in
the first depth category. For those periods for which there was no exact dive number
information (transmissions in which only time spent at depth information was errorfree), the average number of dives from periods with exact dive counts was multiplied
by 6 sec to provide a correction factor for the time at depth.
Data were divided into two periods of the day for diel comparisons. For depth
tags, summary periods
2, 3,
and summary periods 1, 6,
4, and
7,
and
5 (20:00-07:59
8 (08:00-19:59
PDT) were combined for night,
PDT) for day. For duration tags,
16
summary periods 5, 6, 7, and 8 (18:00-05:59 PDT) were combined for night, and
summary periods 1, 2, 3, and 4 (06:00-17:59 PDT) for day.
Statistical Analysis
We used the Statgraphics statistical package for data analysis. Where appropriate,
parametric tests (one-way analysis of variance, with multiple range tests, and t-tests)
were used to test for differences in means. Log transformations of the data were
conducted in cases for which assumptions of parametric tests were violated. Nonparametric tests (Kruskal- Wallis test, and Mann-Whitney U-test) were used to test for
differences between medians when transformations were not successful in correcting
assumption violations. For purposes of clarity, however, we report variable means in
the results, regardless of the tests performed. F and t-statistics are reported for
analysis of variance and t-tests, respectively. We report KW and Z-statistics for
Kruskal Wallis and Mann-Whitney tests, respectively.
RESULTS
Dive habit information was collected for 24 periods; 13 for whale 10836
(representing 786 dives), 1 for whale 830 (121 dives), 4 for whale 834 (332 dives),
and 6 for whale 837 (768 dives).
17
Percentage of Time Submerged
The percentage of time that an animal was submerged during each 3-h summary
period ranged from 91 to 98 % (7= 95.6 + 0.4%, n = 18). Percentage of time
submerged did not differ among animals (KW = 2.8, p = 0.25; Table 1.1), nor did
it differ between night and day (Z = -1.1, p = 0.25).
Total Number of Dives
The total number of dives for a 3-h summary period averaged 103 ± 8, with a
range of 69 to 125 dives. Seventy-one percent of all of these dives were <1 min in
length (Figure 1.1). Total number of dives differed slightly among animals (KW =
5.2, p = 0.07; Table 1.1), with whale 837 having the highest value (5z = 128 ± 17).
Total number of dives did not differ between night and day for any of the whales,
however.
Average Dive Duration
The average dive duration for all whales combined was 1.8 ± 0.1 mins, with a
range of 0.8 to 2.5 min (Table 1.1). Average dive duration differed among whales
(F212 = 4.47, p = 0.04), but not between night and day for any of the whales, when
examined individually.
Table 1.1: Means (± SE's) of dive duration data, by time of day, for all blue whales tagged off central California, 1993.
Significantly different values among whales are denoted by different superscript letters.
WHALE #
all
TIME
all
night
day
% TIME
SUBMERGED
95.6
a 0.4)
all
day
DURATION
OF DIVES
>1 MIN
(min)
103
1.8
5.9
(-± 0.5)
(-± 8)
n= 18
n= 17
n= 16
n= 17
96.1
94
(± 0.7)
n=9
6.0
a 0.4)
(± 7)
2.0
(± 0.2)
n=8
n=9
n=8
95.2
113
1.7
5.8
L 0.6)
a- 15)
n=8
(± 0.2)
a- 0.8)
n=8
n=8
95.9
88
2.1'
6.9
L 0.6)
(± 7)
(± 0.1)
(± 0.6)
n=6
n=5
n=6
n=7
night
DURATION
OF ALL
DIVES
(min)
(± 0.1)
n = 10
10836
TOTAL
NUMBER
OF DIVES
96.0
91
2.1
6.7
(± 1.0)
n=3
(± 10)
a 0.3)
(± 0.9)
n=4
n=3
n=4
95.8
82
(± 0.9)
( 4)
n=4
n=2
2.1
7.4
(± 0.1)
(± 0.2)
n=2
n=2
Table 1.1 (continued): Means (± SE's) of dive duration data, by time of day, for all blue whales tagged off central California,
1993. Significantly different values among whales are denoted by different superscript letters.
WHALE #
TIME
830
all
834
all
night
day
837
all
% TIME
SUBMERGED
DURATION
OF DIVES
> 1 MIN
(min)
96.0
121
1.4
5.0
n=1
n=1
n = 1
96.5
83'
L0.3)
L8)
n=4
n=4
2.2'
7.2'
(± 0.2)
(± 0.7)
n=4
n=4
2.2
7.4
96.5
79
(± 0.5)
(± 8)
( 0.2)
a 1.6)
n=2
n=2
n=2
n=2
96.5
87
2.1
7.1
(± 0.5)
(± 17)
(± 0.9)
n=2
n=2
(± 0.4)
n=2
94.7
128b
1.5b
4.2b
(± 0.2)
n=6
(± 0.8)
n=6
day
DURATION
OF ALL
DIVES
(min)
n=1
L 0.8)
night
TOTAL
NUMBER
OF DIVES
(± 17)
n=6
n=2
n=6
96.0
102
(± 1.0)
(± 23)
1.8
(-± 0.4)
3.8
(-± 0.3)
n=2
n=2
n=2
n=2
94.0
141
1.3
4.5
L 1.1)
(± 22)
a 0.2)
(± 1.2)
n=4
n=4
n=4
n=4
G
20
100
90
80
70
60
50
40
30
20
10
0
I
0-1
1-4
4-7
7-10 10-13 13-16 16-19 19+
Duration category (min)
Figure 1.1: Average number of dives in each of eight duration
categories, during 17 summary periods, for all blue whales tagged off
central California, 1993. Error bars represent standard deviations.
21
Average Duration of Dives > 1 min
The average duration of dives >1 min for all animals and periods ranged from
2.7 to 9.1 min (7( = 5.9 ± 0.5 min; Table 1.1) with considerable variation for each
individual. The durations of these dives differed among whales (F2,13 = 5.85, p =
0.02), but not between night and day for any of the whales, when examined
individually.
Maximum Dive Duration
The duration of the longest dive per period for whale 10836 ranged from 10 to 18
min, with a mean of 13 min a 1 min). There was no difference in this duration
between day and night (Z = -0.74, p = 0.46). Whale 830 reported the underflow
value for duration of longest dive, meaning the maximum dive duration was less than
12 min. Whale 834 also reported underflow values for maximum dive duration for
both its day periods and one night period, while another night period produced a
maximum of 15 min. Whale 837 reported an underflow value for 3 of 6 periods (2
day and 1 night) while its longest dive (15 min) occurred at night, followed by two
values for day (R = 13 ± 1 min). There were not enough data from the duration-only
tags to perform statistical analysis.
22
Maximum Surface Duration
For whale 10836, 85.7% of the summary periods (n = 7) contained maximum
surface durations >15 s (5-( = 55 ± 15 s), with a range from 30 to 90 s. No
difference was found between day and night (t = 0.34, p = 0.75). The duration-only
tags had a 1-min resolution for this variable, and all reported 0's for maximum
surface duration. Thus, there were no surfacings lasting >60 s in the 11 summary
periods reported.
Depth
Whale 10836 was the only animal for which depth information was available. Most
dives were made in the 0-16 m depth bin (R = 50 ± 6 dives/3-h period; Figure 1.2).
The second highest number of dives (R .= 10 + 2 dives/3-h period) were made to 97­
152 m. Overall average depth of dive for a 3-h summary period was 33 m (SE = 2,
range 21-38 m, n = 9). If much of the activity in the uppermost depth bin was
surface activity of some sort (mostly breathing), and only dives in bins >16 m were
considered, the average depth was 105 m (SE = 4, range 84-121 m). Average depths
for all dives did not differ between day and night (Z = -1.69, p = 0.09). Average
depth for dives >16 m did, however, differ between day and night (Z = -2.20, p =
0.03), with those during the day being deeper (5-( = 120 ± 1 m, n = 3) than at night
= 98 + 4 m, n = 6).
23
0-16
17-32
33-48 L
49-96
97-152
percentage of time at depth
153-200
percentage of dives
0
20
40
60
80
100
Percentage
Figure 1.2: Average percentage of dives to each of six depth categories
(n = 9) and average percentage of time spent in each of six depth
categories (n = 7) during 3-h summary periods for blue whale 10836, tagged
off central California, 1993. Error bars represent standard deviations.
24
Depth of the deepest dive was reported for nine summary periods and duration of
the deepest dive for eight summary periods. Of these, six were from the same
summary periods. Therefore we had six instances where the duration of the deepest
dive was known.
For these six periods the average deepest depth was 157 m (SE = 12, range 112­
192) and the average duration of these dives was 10.7 min (SE = 2.0, range 4-18).
Dive duration was not associated with depth of dive (regression, p = 0.84, r =
0.11).
Overall average deepest depth was 151 m (SE = 9, range 112-192, n = 9) with
overall average duration of 10 min (SE = 1.5, range 4-18, n = 8). There was no
difference in deepest depth between day and night (Z = -0.53, p = 0.60).
Time at Depth
Whale 10836 spent most of its time in the 0-16 m depth category (Figure 1.2).
The 16-32 m depth category was the second highest in terms of time spent, followed
by the 48-96 m depth category. Despite 15.2 % of all dives going to the 96-152 m
depth category, whale 10836 spent only 1.2 % of its time there.
Visual Observational Data
Direct observations of blue whale respirations were conducted for nine whales for
a total of 3.3 hr (Table 1.2). Average surface time for all whales was 1.1 ± 0.1 min.
Table 1.2: Comparison of visually observed surface/respiration activities for several species of Mysticete whales.
BLOW
RATE
(blows/min)
BLOW
INTERVAL
Balaenoptera
musculus
0.97
21
Balaenoptera physalus
1.06
13.0
SPECIES
SOURCE
(s)
% TIME
NEAR
SURFACE
4.01
64.8
25.4
This study
-­
-­
--
Kopelman and Sadove,
BLOWS PER
SURFACING
(s)
AVERAGE
SURFACING
1995
13.12
13.6
Stone et al.,1992
(boat present)
(no boat)
0.84
0.81
15
2.8
3.12
49.8
54.6
Megaptera
novaeangliae
1.15
15
3.2
66
25.2
Dolphin,1987
Esrichtius robustus
1.05
13.8
4.2
53.4
22
Wursig et al., 1986
Eubalaena mysticetus
0.77
13.8
13.2
4.3
7.4
71.4
67.2
21
1.28
0.70
15
Dorsey et al.,1989
Wursig et al., 1984
Wursig et al., 1984
123
26
Average number of blows during these surfacing sequences was 4.0 ± 0.2, with
mean blow intervals of 21 ± 1.2 s. Dive times averaged 3.3 ± 0.5 min. Mean blow
rate was 1.0 ± 0.1 blows per mm. Percentage of time in a surfacing sequence
averaged 25.4 ± 1.8.
Average duration of all submergences was 1.0 ± 0.1 min. Submergences <1 min
in length accounted for 78.1 ± 1.7 % of all dives. Maximum dive durations for the
nine whales ranged from 3.2-6.5 min, with an average of 4.7 ± 0.4 min.
Due to the limited number of samples, only two behavior types were described: (i)
feeding (n = 5), characterized by surface lunges, surfacings around feeding birds, or
in visible surface swarms of krill, and (ii) non-feeding (n = 4), characterized by all
other behaviors. There were no differences in any of the respiration variables
between feeding and non-feeding (Z's <1.23, p's >0.22).
Movements
We received five locations for PTT 10836 and two for PTT 834. The minimum
distance travelled from tagging to the last location was 274 km for PTT 10836 and
124 km for PTT 834 (Figure 1.3). These are straight-line distances and represent
minimum distances travelled only. As such, estimates of speeds are also minimums.
After tagging, whale 10836 travelled: west 57 km in 4 h (14.2 km/h), 48 km
southeast in 35.9 h (1.4 km/h), east 56 km in 23.7 h (2.4 km/h), south 32 km in 13.0
27
Guide
Seamount
Figure 1.3: Tagging and satellite-derived locations for whales 10836
and 834. Offshore contours begin at depths of 150 m with subsequent
contours at 50 m intervals.
28
h (2.5 km/h), and 82 km west in 12.3 h (6.6 km/h). All five locations were in water
deeper than 1,000 m.
Following tagging, whale 834 travelled north-northwest 72 km in 59.6 h (1.2
km/h), then south 52 km in 12.9 h (4.0 km/h). Water depths at the first and second
post-tagging locations were 55 m and 370 m, respectively.
DISCUSSION
This study provides the first telemetered information on the dive habits of free-
ranging blue whales, and the first comparing day and night dive data for this species.
The sample size is small, however, due to short-term tag attachment, possibly caused
by hydrodynamic drag from high swimming speeds of blue whales. Additionally, the
40 sec transmission repetition rate combined with blue whales' habit of spending more
than 90 % of their time underwater limited the number of possible transmissions. The
small sample sizes do not lend themselves to rigorous statistical analyses, and so
discussion of trends is most appropriate.
Movements
Although only a few locations were received for each whale, they do convey a
sense of the range, speed, depth of water, and variation of movements among
29
animals. Neither animal stayed in the area in which they were tagged, but ranged
widely off the coast of central California.
The post-tagging locations for whale 834 were both within 40 km of land over the
continental shelf or near the shelf edge (55 m and 370 m water depth, respectively).
Whale 10836 moved 90 km offshore, returning nearshore within 24 km of the
coast, and then moving 100 km offshore again in three days. All post-tagging
locations were in water depths > 1,000 m. The first offshore location was
approximately 25 km southeast of Pioneer Seamount (770 m below sea level) and 15
km northwest of Guide Seamount (1642 m below sea level). Subsequent locations
show the animal over the edge of Monterey Canyon, and directly over the Canyon
itself. Following this, the animal travelled west over the continental rise, an area with
a gently sloping relatively smooth surface, to a location approximately 33 km
southeast of Guide Seamount. Irregular bottom topography, such as created by these
seamounts and Monterey Canyon, may contribute to upwelling and have a major
influence on productivity. Pioneer Seamount typically has increased biological
productivity (Smith et al. 1986). As cetacean food is probably most concentrated in
regions of high productivity (Hui 1985), locations in these regions may not be
coincidental. Great concentrations of zooplankton biomass, particularly E. pacifica
have in fact been found west of Monterey Bay (Huntley et al. 1995).
Calculated minimum travel speeds for whale 10836 were lowest near the
seamounts and Monterey Canyon (< 2.5 km/h). The slower apparent travel speeds in
30
these areas possibly represent feeding or "zig-zag" search patterns, whereas the faster
speeds immediately after tagging and during westward movement over the continental
rise may be more indicative of traveling between feeding areas.
Dive Duration
We found individual variation in the measured dive parameters in this study.
Despite the small sample sizes, significant differences were detected among animals
for total number of dives, average duration of all submergences, and average duration
of dives > 1 min in length. Such individual variation has been reported for North
Atlantic right whales (Nieukirk and Mate in prep.). Despite possible differences in
diving strategies, there were no significant differences in percentage of time
submerged between animals, with all >94 %.
When the animals are considered separately, three diving strategies can be
described. Whale 834 exhibited frequent, shorter duration dives during the day, with
fewer, but longer dives at night. Whale 837 dove most frequently, but for longer
duration during the day than at night. Whale 10836 dove most frequently, and for
shorter duration at night. During the day, whale 10836 dove less frequently, but dives
were of longer duration than at night. Whale 830 also made many, short-duration
dives during night. No daytime periods were available for this animal.
The daytime pattern of whale 834 may be indicative of surface-traveling or
socializing behavior. Lunge-feeding on surface-swarming prey, which we observed on
31
several occasions, may also result in this dive pattern. Dorsey et al. (1989) reported
that dives of surface-feeding bowhead whales were shorter than those of bottomfeeding bowheads or those engaged in non-feeding activities. Surface-feeding fin
whales off Long Is la0, New York also exhibited shorter dive durations than nonsurface-feeding fin whales (Kopelman and Sadove 1995). The lunge-feeding blue
whales we observed were consuming T. spinifera. T. spinifera is a neritic species
which is not known to migrate vertically, occurring almost entirely at depths of <100
m (Boden 1955, Brinton 1962). Adults of this species aggregate in conspicuous
daytime surface swarms along the California coast during spring and summer (Smith
and Adams 1988). The frequent, short dives of whale 834 may represent feeding
more often during the day, taking advantage of such surface swarms. Our location
data suggest this animal was over the continental shelf or near the shelf edge during
the entire time it was tracked, and its' progress through this area was quite slow (1.2
km/h).
The longer daytime dives of whale 837 may represent feeding at depth, traveling,
or searching the water column for prey. Wursig et al. (1986) reported longer dives for
benthic-feeding gray whales than for non-feeding whales, and suggested duration of
dive may be a useful indicator of feeding. Dorsey et al. (1989) reported longer dive
durations for bottom-feeding bowhead whales than surface feeders. A positive
correlation between dive duration and depth was also shown for feeding humpback
whales in Alaska (Dolphin 1987).
32
The frequent, short-duration nighttime dives of whales 10836 and 830 may
indicate foraging near the surface on vertically migrating prey. Right whales off Cape
Cod engaged in shorter dives at night during a time in which their zooplankton prey
underwent strong diel vertical migration (Winn et al. 1995). While we observed blue
whales feeding on surface swarms of T. spinifera, other krill species have been
described as part of blue whale diets. Adults of E. pacifica are common offshore in
the California current (Brinton 1962) and migrate vertically from daytime depths of
up to 280 m to nighttime depths between 0 and 140 m (most above 80 m). As a
result, blue whales may preferentially feed at night when some types of krill are
shallower and more accessible. The dives of whale 10836 were consistent with a
pattern that might be expected if it were feeding on E. pacifica offshore. The Argos
locations from this animal were approximately 100 km offshore, in water depth
>3,000 m. Four of the five locations were in traditionally productive areas near
seamounts or the Monterey Canyon. Movement through these productive areas was
quite slow (< 2.5 km/h), allowing time for feeding.
Dive Depth
The overall distribution of dive depths for whale 10836 revealed a bimodal
distribution, with most dives between 0-16 m and 96-152 m. The submergences in the
first 16 m may represent respirations during surface intervals between longer dives.
33
More than 71 % of all dives were <1 min in duration and <16 m deep. Surfacefeeding would also contribute to submergences in the first 16-m depth range.
Dives to the 96-152-m depth range were the second most frequent (15 % of all
dives) and took place in water > 1,000 m. Such dives may reflect searching the
water column for prey or feeding at depth. The average depth of dives > 16 m was
lower at night than during the day, supporting the possibility that the whale may have
been feeding nearer the surface at night on vertically migrating prey.
Although whale 10836 dove to depths of 200 m, and regularly up to 152 m, it
spent very little time overall at these depths (<1% and 1.2%, respectively). This
could be indicative of a pattern of spike dives where the animal spends more of its
time descending or ascending searching for food versus swimming along at depth. A
similar pattern was seen in pygmy blue whale (Balaenoptera musculus brevicauda)
dive traces from a depth sounder in the Indian ocean (Ailing et al. 1991). In several
instances, the pygmy blue whales appeared to descend below the deep scattering layer
and then ascend through it. Right whales east of Cape Cod have been reported
occasionally diving deeper than their main prey patch, possibly as a means of
detecting changes in depth distribution of prey patches (Winn et al. 1995).
There was no evidence of a correlation between dive duration and depth for tag
data, but this may be a result of small sample sizes rather than a true characteristic of
blue whale dive habits.
34
Tag Data vs Visual data
Due to the limited nature of the data collected, direct comparisons could not be
made between all the variables obtained from tagged whales and those obtained from
visual observations.
Overall dive durations were shorter from visual observations than those from
tagged whales, as were durations of dives > 1 min. The percentage of submergences
that were <1 min in length was only slightly higher for whales observed visually than
for tagged whales (78% vs 71%). The maximum dive durations reported for the nine
visually observed whales were much shorter than that reported for tagged whales.
This may be misleading, however, in the cases of whales 830, 834, and 837, for
which underflow values were reported for the majority of the summary periods. All
these "differences" are likely due to either small sample sizes or the limitations of
visual observation to reacquire whales which swam out of visual range on long dives.
Most estimates of mean dive duration from visual observations are biased downward,
as whales resurfacing after a long dive can be farther away, and thus more difficult to
find and recognize than after short dives (Dorsey et al. 1989). Harvey and Mate
(1984) discovered similar bias in their analyses of visual vs. VHF radio tag data for
gray whales (Esrichtius robustus).
No significant differences were found in the data obtained from visual
observations between feeding and non-feeding blue whales, although this may be a
result of the very small sample sizes.
35
Comparative Discussion
Dives <1 min in duration likely represent blow intervals during surfacings
between longer "true" dives. Thus comparison of dives >1 min in length is most
meaningful.
Dive habit studies conducted during whaling operations reported dive durations for
blue whales ranging from 10-50 min (Laurie 1933, Zenkovich 1936, and Tomlin
1967). The longer dives could be an artifact of whale behavior, with blue whales
exhibiting avoidance behaviors when chased by whaling ships. Doi (1974), Yablokov
et al. (1974), and Lockyer (1976) also used whaling data, but reported much shorter
dive durations for blue whales (5.7, 2-5, and 2-7 min, respectively). These values are
much closer to the 5.9 min average dive duration from our tagged whales and the 3.3
min average durations from our visual observations.
Dive durations from this study are in close agreement with recent visual and
telemetry studies of other free-ranging whale species: 2.88 min for summering north
Atlantic fin whales (Balaenoptera physalus) with boats in the area, and 3.35 min
without boats (Stone et al. 1992); 2.66 min for surface-feeding fin whales and 3.10
min for non-surface-feeding fin whales off eastern Long Island (Kopelman and
Sadove 1995); 4.85 min for a fin whale in Iceland (Watkins et al. 1984); 3.18 min for
summering gray whales (Wursig et al. 1986); 3.0 min for humpbacks (Megaptera
novaeangliae) (Dolphin 1987); 4.42 min for summering bowhead whales (Eubalaena
mysticetus) in 1980-1984 (Dorsey et al. 1989); 3.43 min in 1980 and 1981, and 12.08
36
min in 1982 (Wursig et al. 1984); 2.12 min for right whales off Cape Cod (Winn et
al. 1995).
The percent of the surface-dive cycle spent near the surface (25.4 %) was very
similar to many previously-studied species, with the exception of fin whales (Table
1.2). This latter difference is likely related to the way in which the data were
collected rather than representing a biological difference among species. Most studies,
including ours, defined a true dive to be any submergence > 1 min, while Stone et al.
(1992) used 25 sec. This would lead not only to shorter surface durations and hence
smaller percentages of time spent at the surface, but also to shorter dive durations, as
was evidenced for fin whales.
This problem is further emphasized when considering percentage of time spent at
the surface obtained from our tagged whales. The tags were programmed to record
any submergence > 6 sec as a dive. As such, the average percentage of time
submerged for blue whales was 95.6, or 4.4% of time at the surface. The percentage
of time a whale spends at the surface is a useful value in estimating population size
from line-transect surveys (Hiby and Hammond 1989). The time during which whales
are visible for "sighting" differs depending upon the survey method. In aerial surveys,
a whale in clear water is often detectable before and after it surfaces to breathe when
it is close to the trackline. Thus, including all or part of the blow interval in the
calculation of a sighting correction factor may be appropriate for an aerial survey. In
ship-board surveys, whales below the surface are more difficult to detect than from an
37
airplane, making absolute surface time a more accurate estimator of a sighting
correction factor.
For future studies we recommend collecting information on individual dives in
addition to summary information so direct comparison can be made between the
depths and durations of all dives, and a biologically-meaningful differentiation
between blow intervals and true dives can be determined. Individual dive information
would also allow for examination of the shape of the dives.
This study is the first to provide detailed dive habit information via radio tags for
free-ranging blue whales. It provides preliminary evidence for individual variability
and some diel variation in blue whale diving habits. It also demonstrates the feasibility
of radio-tagging and satellite-monitoring this endangered species.
38
CHAPTER 2
Dive Characteristics of Satellite-Monitored Blue Whales
(Balaenoptera musculus) off the Southern California Coast
Barbara A. Lagerquist and Bruce R. Mate
Department of Fisheries and Wildlife
Oregon State University
Corvallis, Oregon
39
INTRODUCTION
Blue whales are commonly seen in southern California waters in June September
(Barlow 1995), and some evidence suggests their numbers have increased in this area
between 1979/80 and 1991 (Barlow 1994). During the summer months, wind-driven
upwelling in the California Current waters contributes to high productivity, resulting
in the use of these waters as feeding grounds for blue whales. Irregular bottom
topography and islands, such as the Channel Islands within the Southern California
Bight, contribute to locally intense, highly variable mixing and upwelling (Smith et
al. 1986). This phenomenon is intensified in the fall months by interaction with the
Davidson Counter Current. Concentrations of feeding blue whales in these nearshore
areas provide an excellent opportunity for behavioral studies of this endangered
species.
In determining whale abundance from sighting surveys, correction factors account
for animals that are not detected during the survey. Whales may be missed by
observers: 1) even when available to be seen (perception bias), or 2) because they
were underwater or surfaced behind a swell (availability bias) (Marsh and Sinclair
1989, Barlow 1995). Estimates of g(0), the probability that a group of whales on the
transect line will be seen (Barlow 1994), should include both types of biases.
Knowledge of surfacing rates of the species in question is necessary for the correction
of availability bias (Doi 1974, Hiby and Hammond 1989, Hiby 1992, Barlow 1995),
emphasizing the need for studies in which this type of data is collected.
40
Satellite-monitored radio tags can provide information on the surfacing rates of
presumably undisturbed, free-ranging whales, as well as information on the duration
and depths of their dives. Satellite telemetry studies can provide such information for
a number of animals simultaneously, both day and night, over large geographical
areas. The time a cetacean spends at the surface is variable and somewhat species-
specific (Smith et al. 1986). Thus, estimates of g(0) are generally species and area
specific (Wade and Gerrodette 1993). Variation in surfacing rates also occurs among
individuals of the same species and within individuals at different times (Joyce et al.
1989, Joyce et al. 1990, Nieukirk and Mate in prep.; Lagerquist et al. in prep).
To be of most value, information from tagged animals should be correlated with
observed surfacing behavior as a means of 'ground-truthing' tag data (Leatherwood et
al. 1982). Additionally, visual observations of untagged whales may be valuable in
demonstrating whether tagged whales exhibit 'abnormal' behavior, possibly as a result
of tagging.
The purpose of this study was to characterize the dive habits of visually-observed
and radio-tagged blue whales and determine whether individual differences in diving
behavior, or diel or geographical differences have a significant effect on the amount
of time blue whales spend at the surface.
41
METHODS
Two blue whales were tagged in October 1994 (tag numbers 848 and 23029) and
six between 8 August and 2 October 1995 (tag numbers 837, 841, 845, 2082, 2083,
and 23031) with satellite-monitored radio transmitters. All whales were tagged in the
Santa Barbara Channel, California. Tags were deployed 1-4 m behind the blowholes
with a 68-kg crossbow, from a 5.3-m rigid-hulled inflatable boat.
Tags consisted of a Telonics (Mesa, AZ) ST-6 Argos transmitter, a Wildlife
Computers (Woodinville, WA) controller board, and a pressure transducer. The tag
housing was a stainless steel cylinder that was 5.4 cm in diameter by 15.6 cm in
length, weighed 0.81 kg, and included two subdermal attachments. Attachments
consisted of stainless steel rods (16.5 cm long, 6 mm in diameter) with a pair of
double-edged blades at the distal end. One pair of folding toggles was mounted behind
the blades to prevent outward migration of the tag. The tag was filled with carving
wax to reduce air spaces and add structural strength. A flexible 18-cm whip antenna
was mounted in one plastic end-cap perpendicular to the tag housing. The tags were
powered by eight Duracell® 2/3-A lithium batteries.
The UHF transmitters emitted 400-mW signals to Argos receivers on three NOAA
TIROS-N weather satellites in sun-synchronous polar orbits. The transmitters were
programmed to transmit on a schedule coinciding with the satellite schedules over the
study area. During each transmission schedule, the tags transmitted every 10 sec when
at the surface. A saltwater conductivity switch was used to assure the tag was at the
42
surface before initiating a transmission. This switch determined the duration of
surfacings and dives. A dive was defined as any submergence greater than 6 sec. Tag
locations were calculated by Service Argos from Doppler shift data when two or more
messages reached a satellite during one pass (Argos 1990).
The tags recorded information on every dive in a day and stored this information
for transmission the following day. Data were stored daily as both individual dive
information and summary information for each of eight 3-h periods. The 960-ms
transmissions consisted of a discrete identification code and 256 bits of sensor data,
including cyclic redundancy checks (CRC) for error detection. Each 256-bit message
contained the following summary information: total number of dives; total time at the
surface (64-sec resolution); maximum time at the surface (MSURF; 1 sec-resolution if
< 60 sec, 4-sec resolution if > 60 sec); maximum dive duration (MDIVE; 16-sec
resolution); and maximum depth of the deepest dive (MDEPTH; 8-m resolution).
Messages also contained the following information for 11 consecutive surface/dive
events: surface duration (SURF; 0.5-sec resolution if <8 sec, 32-s resolution if > 8
sec); duration of the subsequent dive (DIVE; 4-sec resolution if < 64 sec, 32-sec
resolution if > 64 sec); and maximum depth of the dive (DEPTH; 2-m resolution if
<8 m, 8 m resolution if > 8 m). Henceforth, maximum depth of the deepest dive
during a summary period and maximum depth of individual dives will be referred to
as maximum depth and depth, respectively. Transmissions cycled through both the
summary period information, and the individual surface/dive events of the previous
43
day. The first eight transmissions in a day contained data from the eight summary
periods of the previous day, as well as data from the first 11 surface/dive events in
those summary periods. The second eight transmissions contained the same summary
period data as the first eight transmissions, in addition to the next 11 surface/dive
events in those periods.
Visual Observations
Whale respiration patterns were observed from a 16.6 m vessel (R/V 'Cille) using
the focal animal sampling technique (Altmann 1974). Before beginning an
observation, general behavioral observations and identification photographs of the
whales were taken. If more than one whale was present, the most easily
distinguishable animal was chosen for sampling. Observation distances varied from
10-150 m.
The desired duration for each sampling period, or 'bout' was 30 min. Bouts were
terminated prior to 30 min in cases where the focal animal could not be identified,
either due to poor weather conditions, losing track of the focal whale, or the
affiliation of other whales with the focal animal. The primary observer called out
sightings to a second person who recorded exact times (h:min:sec) of exhalations and
behaviors. In order to differentiate between "true" dives and blow intervals, we
performed log-survivorship analysis (Machlis 1977, Fagen and Young 1978, Slater
and Lester 1982) on the recorded submergences. The breakpoint between the two
44
types of submergences was objectively determined using the method described by
Beavers and Cassano (1996). Because the log-survivorship curve could not be
described fully by a negative exponential function, the data were modeled as
overlapping chi-square and lognormal distributions. The submergence duration
beyond which the tail areas of the overlapping distributions were the same was then
selected as the breakpoint between the two behavior types.
Four variables similar to those measured in other respiration studies (Stone et al.
1992, Kopelman and Sadove 1995) were evaluated: dive time (DT); duration of
surfacing sequence between successive dives (surface interval, ST); number of blows
during the surface interval (NB); and the mean time between blows (mean blow
interval, BI). Blow rate was calculated as the number of blows per minute for each
whale. The percentage of time spent in a surface interval (PRCT_ST) during a
dive/surfacing cycle was calculated as surface interval time divided by the sum of dive
time and surface interval time, and multiplied by 100. The percentage of time for
which any portion of the whale's body was visible at the surface (PRCT_SURF) was
calculated as the total amount of time between every submergence divided by the total
observation time, and multiplied by 100.
Two methods of analysis were employed to test for differences in variables among
behavior categories. For the purpose of comparison with other respiration studies
(Wursig et al. 1984, Wursig et al. 1986, Dolphin 1987, Dorsey et al. 1989, Stone et
al. 1992, Kopelman and Sadove 1995), data from all observation bouts were pooled
45
for each behavior category. That is, individual whales contributed more than one data
point to each behavior category. Conclusions based on pooled data sets should be
viewed with caution, however, as such techniques can lead to underestimation of
standard error terms, and the probability of rejecting a true null hypothesis is likely to
be substantially greater than the stated alpha level (Mach lis et al. 1985). To avoid the
problems of pooled data sets, analyses were performed a second time using mean
values for the variables from each observation bout. Mean blow rates and percentage
of time for which an animal's body was visible at the surface were analyzed using the
second technique only, as only one rate and percentage was calculated for each
observation bout.
Tag Data
Log-survivorship analysis, similar to that described above, was also used on the
tag data to distinguish between blow intervals and true dives. In this case the data
were modeled as two overlapping lognormal distributions, and the breakpoint was
determined in the same way as for visual observation data. Similar dive variables
were then calculated.
Blow rates (blows/min) for tagged whales were calculated from summary period
data, by dividing total number of dives during a summary period by 180 min (3 h).
Percentage of time at the surface (PRCT_TSURF) was calculated as the total time at
46
the surface during a summary period divided by total summary period time (3 h), and
multiplied by 100.
Statistical Analysis
The Statgraphics statistical package was used for data analysis. Where appropriate,
parametric tests (one-way analysis of variance, with multiple range tests, and
multifactor analysis of variance) were used to test for differences in means. Log
transformations of the data were conducted in cases for which assumptions of
parametric tests were violated. Non-parametric tests (Kruskal-Wallis test) were used
to test for differences between medians when transformations were not successful in
correcting assumption violations. For purposes of clarity, however, means are
reported in the results, regardless of the tests performed. F and KW test statistics are
reported for analysis of variance and Kruskal -Wallis tests, respectively. Standard
deviations accompany overall means, whereas standard errors are reported for means
of different factor levels in statistical tests. Spearman Rank correlation analyses were
conducted to test for associations between variables, and r-values are reported in such
cases.
47
RESULTS
Visual Observations
Respiration data were collected on
25
individual whales from a total of 8.75 h of
visual observation. Behaviors were classified into four categories for analysis:
feeding, resting, milling, and traveling. Feeding (n =
6)
was characterized by surface
lunges, surfacings around feeding birds, or in visible swarms of hill. Resting whales
(n =
3)
were those animals that remained motionless at the surface, or surfaced in the
same general location repeatedly with the same orientation and no forward movement.
Milling whales (n =
6)
changed their direction of travel repeatedly, but stayed in the
same general area over the period of observation. Traveling whales (n = 4)
maintained a relatively constant direction of travel over the course of observation. In
six of the 25 observation episodes, general whale behavior was not recorded, and thus
were not included in the behavioral analysis.
Pooled Data Analyses
The mean duration of all submergences was 40.2 sec (SD =
sec, n =
70
686).
92.8,
The mean duration of all surfacings was 6.4 sec (SD =
range
1-667
3.9,
range 1­
sec, n = 715). A submergence duration of 24 sec was selected as the breakpoint
between true dives and interblow intervals (Figure
2.1).
3.0
2.5
2.0
44.
1.5
9_.
I'.
1.0
0.5
0.0
-0.5
-50
1
0
I
I
I
i
I
I
I
I
I
I
I
I
I
I
I
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800
Time (s) between surfacings
Figure 2.1: Log-survivorship plot of all times between surfacings for 25 untagged blue whales
observed off southern California, 1995.
49
Overall mean true dive duration was 197.6 sec (SD = 159.9, range = 26-667
sec, n = 102). True dives differed among feeding (5-( = 142.5 ± 20.8 sec, n = 40),
and both resting (5-( = 251.4 ± 25.8 sec, n = 13), and milling (R = 276.4 ± 52.3
sec, n = 18) whales (F3.85 = 3.77, p = 0.01), but not among these behaviors and
traveling (R = 202.7 ± 37.2 sec, n = 18) whales.
Overall mean surface interval was 88.2 sec (SD = 65.4; range = 3-339 sec; n =
118). Surface intervals differed among feeding (R = 72.5 ± 7.5 sec, n = 45) and
milling (R = 123.7 ± 19.0 sec, n = 21) whales (F3,96 = 3.12, p = 0.03), but not
among these behaviors and traveling (R = 80.9 ± 19.1 sec, n = 18) and resting (R
= 87.8 + 9.1 sec, n = 16) whales.
Overall true dive durations and surface intervals were positively correlated (r =
0.50, p <0.001, n = 87). Surface intervals were more closely related to the duration
of the previous dive (r = 0.54, p< 0.001, n = 98) than to the duration of the
following dive (r = 0.40, p <0.001, n = 93).
Overall mean number of blows during a surface interval was 5.5 (SD = 3.8,
range = 1-18, n = 119). Number of blows differed among milling (7 = 7.4 ± 1.00,
n = 22) and both resting OR = 4.8 ± 0.4, n = 16) and feeding (R = 4.6 ± 0.5, n =
45) whales ( F3.97 = 3.05, p = 0.03), but not among these and traveling whales (5z =
5.1 ± 1.1, n = 18). Number of blows per surface interval was highly correlated with
the duration of a surface interval (r = 0.96, p <0.001, n = 87).
50
Overall mean blow interval was 12.3 sec (SD = 3.6, range = 3.5-24 sec, n =
119). Traveling whales had different blow intervals than whales exhibiting other
behaviors (F3,94 = 5.77, p = 0.001). Mean blow interval was higher for traveling
whales (R = 15.3 ± 1.0 sec, n = 18) than for feeding (R = 12.0 ± 0.5 sec, n =
43), milling (R = ± 11.8 ± 0.6 sec, n = 21), or resting whales (T( = 10.8 ± 0.9
sec, n = 16). Blow interval was negatively correlated with dive duration (r = -0.28,
p= 0.01, n = 87).
Overall mean percentage of time spent in a surface interval was 33.1 % (SD =
18.0, range = 2.2-89.9, n = 98). There was no difference among the behavior
categories (F3,82
= 2.02, p = 0.12).
Analysis of Means
Overall mean blow rate was 1.6 blows per minute (SD = 0.7, range = 0.7-3.4, n
= 25). Overall mean percentage of time for which an animal's body was visible at the
surface was 16.6 % (SD = 7.7, range = 6.1-36.2, n = 25). There were no
significant differences among behavior categories for any of the visual observation
means (p's >0.10; Table 2.1). Trends, similar to the differences noted in the pooled
data sets, were apparent, however. Due to small sample sizes, analysis of means tests
may not have had the required power to detect differences among behavior categories.
Thus, examination of trends may be most appropriate.
51
Table 2.1: Means ± SE's of visual observation variables, by behavior category, for
untagged blue whales off southern California, 1995.
SURFACE
INTERVAL
FEED
MILL
REST
TRAVEL
DIVE
DURATION
(s)
(0
89.7
188.5
± 19.9
a=6
+ 48.3
n=5
BLOW RATE
% TIME IN
SURFACE
INTERVAL
NUMBER
OF
BLOWS
5.7
BLOW
INTERVAL
11.7
1.5
36.3
± 1.0
+ 0.9
n=6
+ 0.2
n=6
+ 3.7
n=5
n=6
(blows/min)
(s)
122.6
318.9
7.4
11.7
1.4
37.2
+ 21.3
+ 93.7
a=5
+ 0.4
a=6
+ 0.2
n=6
+ 4.8
a=6
± 1.2
n=6
5.2
11.5
±7.7
±5.8
±0.7
a=3
a=2
n=3
+ 1.2
n=3
92.7
257.2
1.2
n=4
26.7
±0.3
±0.3
n=3
n=2
97.5
215.3
6.1
15.0
1.1
24.0
+ 50.2
+ 82.2
+ 2.8
n=4
n=4
+ 0.2
n=4
+ 6.3
n=4
+ 1.4
a=3
n=4
Tag Results
A total of 488 messages were received from the eight tagged whales (Table 2.2).
Of these, 37.3 % (182 messages) contained errors and were not included in the
analyses.
Summary information was received for 203 3-h periods (Table 2.3). Individual
animals differed from one another with respect to all summary variables (p's
<0.001).
Duration and depth information was received for 1,822 individual dive events
(Table 2.3). Durations and depths of all submergences were correlated (r = 0.41, p
<0.001). Surface durations, submergence durations, and depths of submergences all
Table 2.2: Deployment dates, number of days for which messages were received, and number of messages received for the
eight blue whales tagged off southern California, 1994/95. Number of dive events is the number of error-free events in the
messages used in analysis.
PTT
Deployment
Date/Time (GMT)
# Days
Messages were
received
# Messages
received
# Messages
used in
analysis
# Dive
Events
00837
9/19/95
22:28
13.2
135
88
491
00841
9/10/95
22:41
16.2
167
108
641
00845
9/10/95
23:40
2.1
22
9
51
00848
10/6/94
18:58
1.4
33
20
126
02082
10/2/95
18:14
39.4
57
40
294
02083
8/8/95
22:20
25.1
32
13
23029
10/7/94
17:27
0.8
11
5
49
23031
10/2/95
19:10
5.1
31
20
132
103.2
488
306
1822
Total
38
.
Table 2.3: Means ± SD's of all dive variables for all blue whales tagged off southern California, 1994/95.
N
% TIME
AT SURFACE
203
1822
MAXIMUM
SURFACE
DURATION
MAXIMUM
DIVE
DURATION
MAXIMUM
DIVE
DEPTH
(s)
(s)
(m)
2.7
0.7
8.3
675
±2.0
±0.4
±16.3
±436
SURFACE
DURATION
DIVE
DURATION
DEPTH
TRUE DIVE
DEPTH (m)
(s)
(s)
2.6
128
18
35
+ 3.3
+ 234
+ 37
+ 55
NUMBER
OF BLOWS
BLOW
INTERVAL
TRUE DIVE
DURATION
(s)
548
BLOW RATE
(blows/min)
SURFACE
INTERVAL
(m)
(s)
(s)
316
22.2
2.0
±316
±31.8
±1.5
19
±5
177
±85
% TIME IN
SURFACE
INTERVAL
10.7
±15.4
54
differed among animals (p's <0.001). When examined individually, correlations also
existed between durations and depths of all submergences for seven of eight whales
(r's ranging from 0.23-0.91, p's <0.01, n's ranging from 38-641).
A submergence duration of 32 seconds was selected as the breakpoint between
true dives and interblow intervals and resulted in a sample of 548 true dives (Figure
2.2). Variables analogous to those from visual observations were then calculated
(Table 2.3). Number of blows per surface interval was correlated with surface interval
time (r = 0.89, p <0.001, n = 548). Mean blow intervals were negatively correlated
with true dive duration (r = -0.20, p = 0.002, n = 231) and number of blows per
surface interval (r = -0.23, p <0.001, n = 231). True dive durations and depths of
true dives were correlated (r = 0.55, p <0.001). All variables differed among
animals (p's <0.001).
When examined individually, all whales exhibited positive correlations between
surface interval time and number of blows per surface interval (r's ranging from 0.62­
0.97, p's <0.04, n's ranging from 6-194). Whales 837, 848 and 23031 exhibited
correlations between surface intervals and the previous true dive duration (whale 837 r
= 0.29, p = 0.001, n = 138; whale 848 r = 0.55, p = 0.009, n = 24; whale 23031
r = 0.35, p = 0.05, n = 31). Whale 848 also exhibited a positive correlation
between surface interval and duration of the subsequent dive (r = 0.66, p = 0.04, n
= 11), as did whales 2082 (r = 0.36, p < 0.001, n = 161) and 2083 (r = 0.86, p
= 0.02, n = 9). Durations of successive true dives were positively correlated for two
4
3
k. %se_-....
41,
*
2
1
0
I
r
I
I
I
I
I
1
I
0
200
400
600
800
1000
1200
1400
1600
Time (s) between surfacings
Figure 2.2: Log-survivorship plot of all times between surfacings for eight blue whales tagged
off southern California, 1994/95.
1800
56
whales (whale 841 r = 0.31, p = 0.008, n = 76; whale 2082 r = 0.21, p = 0.007,
n = 161). Mean blow intervals were negatively correlated with true dive duration for
four whales (whale 837 r = -0.30, p = 0.009, n = 78; whale 841 r = -0.29, p =
0.02, n = 66; whale 848 r = -0.47, p = 0.05, n = 18; whale 23031 r = -0.54, p =
0.01, n = 24). In the case of whale 848, mean blow interval was also negatively
correlated with surface interval time (r = -0.51, p = 0.03, n = 18) and number of
blows per surface interval (r = -0.63, p = 0.01, n = 18). All but one whale
exhibited correlations between true dive durations and depths of true dives (r's
ranging from 0.58-0.79, p's <0.01, n's ranging from 19-229).
Diel comparisons
Whales 845 and 23029 were not included in time of day comparisons for the
whales as a group, because they did not provide data for all time periods. Whale
23029 was also excluded from individual diel comparisons as it provided data from
daytime only. Whale 2083 was excluded from group comparisons of dive variables
analogous to those from visual observations, because no data were provided for
twilight.
After controlling for individual differences, some summary variables also
exhibited significant diel differences. The maximum surface duration during a
summary period was higher at night (R = 10.4 ± 2.5 sec, n = 76) than during either
day (5-z = 4.7 ± 2.7 sec, n = 75) or twilight (R = 5.5 ± 3.2 sec, n = 45; F2,178 =
57
5.41, p = 0.005). Maximum dive duration during a summary period was lower at
night (5z = 563 ± 62 sec, n = 76) than either daytime (5- = 800 ± 68 sec, n = 75)
or twilight (R = 715 ± 81 sec, n = 45; F2,178 = 5.86, p = 0.003). Percentage of
time at the surface during a summary period was higher at night (R = 3.1 ± 0.2 %,
n = 76) than during the day (5z = 2.5 ± 0.2 %, n = 75) or at twilight (R = 2.4 ±
0.3 %, n = 45), but these differences were not significant (F2,178 = 1.26, p = 0.29).
Diel variation in maximum dive depth differed depending on the whale in question
(Table 2.4), as evidenced by a significant interaction between time of day and
individual whale (F10,178 = 23.36, p <0.001). All whales, with the exception of
whale 2083, had shallower maximum dive depths at night, however, than at other
times of the day. These differences were significant in the cases of whales 837 (KW
=44.90, p <0.001) and 848 (F2,6 = 24.96, p = 0.001). Maximum dive depth for
whale 2083 was shallowest during twilight. Both twilight and night dives for this
whale were of shallower maximum depths than dives during the day (F
s 2,10 = 7.41, p
= 0.01).
Significant interactions existed between individuals and time of day for surface
durations, submergence durations, and depths of submergences (p's <0.005), thereby
prohibiting examination of diel variation for all whales combined. Diel comparisons
of the above-mentioned variables were therefore conducted for each animal separately
(Table 2.5).
Table 2.4: Means ± SE's of summary period variables, by time of day, for all blue whales tagged off southern California,
1994/95. Within each variable for each whale, different superscript letters refer to significant diel differences.
837
WHALE /
% TIME AT
Day
SURFACE
Night
Twilight
BLOW RATE
(blow/min)
Day
Night
Twilight
MAX. SURFACE
Day
DURATION (s)
Night
Twilight
MAX. DIVE
DURATION (s)
Day
Night
2.0
841
3.1'
±0.2
±0.4
n = 21
n = 30
2.5
5.4b
845
4.2
848
5.2
2082
0.7
2083
2.1
±0.1
±0.0
±0.3
±0.4
n=2
n=4
n = 12
n=3
n=2
n=5
4.2
5.4
0.7
±0.5
±0.1
±0.2
n = 21
n = 22
n=2
n=3
n = 17
n=7
2.0
3.9'
±0.5
0.7
0.9'
±0.1
±0.1
0.8
1.2b
±0.0
±0.1
0.7
1.1'"
±0.1
4.6'
±0.7
7.0
±1.4
3.9
7.8'
18.0
±2.3
4.3'
11.1'
±0.5
± 1.7
707'
+ 38
789'
+146
487b
+ 22
±0.2
±0.2
n=2
n=8
n=3
0.8
1.0
0.3
±0.1
±0.1
1.0
±0.0
0.9
±0.1
0.8
±0.2
5.0
±0.0
4.8
±0.5
0.3
±0.1
0.2
±0.1
2.9
±0.2
8.5
8.0
15.8
±2.5
±2.5
±12.6
4.5
2.5
±0.1
n=6
2.4
1.9
±0.6
±0.1
±0.1
±1.1
0.5
2.4
2.0
±0.3
n = 14
2.0
±0.4
±0.5
±0.2
7.1
23031
±0.9
±0.2
n = 14
23029
0.5
±0.0
±0.3
n=4
0.9
±0.0
0.5
0.5
±0.0
±0.0
0.6
0.5
±0.1
±0.1
3.7
±0.3
0.5
±0.1
6.5
±0.5
4.6
±0.3
8.3
4.1
±1.8
±0.7
6.8
3.7
± 1.6
±0.3
±0.5
±0.2
584'
+ 11
957
904
440
859
+ 104
+ 194
+ 187
+0
+ 81
366b
408
435b
862
563
664
+ 30
+ 64
+ 19
+ 77
+ 19
+ 64
528
Table 2.4 (continued): Means ± SE's of summary period variables, by time of day, for all blue whales tagged off southern
California, 1994/95. Within each variable for each whale, different superscript letters refer to significant diel differences.
WHALE #
Twilight
MAXIMUM
837
841
770°
571°
±96
845
±77
238
152
848
2082
640'
960
±40
±186
236
96
Day
250°
±1
±4
±100
±6
±19
Night
63°
234
116
III°
68
DIVE DEPTH
2083
23029
756
589
±37
±43
172
±12
23031
220
±8
252
±0
(m)
Twilight
±7
222°
±11
±3
238
±4
±48
±12
188
32
±11
80
±11
83"
219
±17
±33
63°
±18
252
±0
Table 2.5: Means ± SE's of individual dive variables, by time of day, for all blue whales tagged off southern California,
1994/95. Within each variable for each whale, different superscript letters refer to significant diel differences.
837
WHALE i
SURFACE
DURATION
Day
(s)
Night
Twilight
DIVE
Day
DURATION (s)
Night
Twilight
DIVE
Day
DEPTH (m)
Night
Twilight
841
845
848
2082
2083
23029
23031
1.6'
2.1'
6.1
1.3
3.06
10.03
2.72'
±0.1
±0.1
±0.2
±0.8
±0.1
±0.09
±1.20
±0.16
n = 160
n = 209
n = 21
n = 59
n = 85
n=8
n = 49
n = 47
2.8
2.81'
2.0°
3.11,
2.3
1.4
2.68
±0.1
±0.3
±0.3
±1.2
±0.1
±0.14
±0.20
n = 169
n = 157
n = 27
n = 37
n = 130
n = 22
n = 36
7.4
1.0
2.8°
5.3
1.4
2.15
±0.1
±0.4
±1.1
±0.1
±0.64
±0.20
n = 130
n = 108
n = 30
n = 63
n=5
n = 26
2.001'
77'
132'
49
57
325
32.50
48.25
142.51
± 12
± 18
± 16
± 14
± 39
± 4.27
± 13.44
± 34.27
85'
40b
53
63
311
38.09
137.00
+ 10
+5
+ 18
+ 17
+ 27
+ 3.92
+ 31.07
177°
63b
81
410
30.80
99.15
+ 31
+ 11
+ 31
+ 43
+ 9.07
+ 36.97
22'
±4
10°
±1
31'
±5
23'
±3
7°
±2
22
±4
14
±2
9
±3
16
±5
10
±3
22
±8
17"
±4
11'
±1
23b
±4
10.50
25.71
25.26
±3.05
±6.20
±7.95
13.55
11.89
±2.23
±3.24
16.20
±9.56
15.62
±8.58
61
Surface durations were longest at night for five of the seven whales examined.
Whales 837 and 841 had longer surface durations at night and twilight than during the
day (837 F2,456 = 6.69, p = 0.001; 841 F2,471 = 6.12, p = 0.002), while 23031's
surface durations were longer at night and day than during twilight (F2,106 = 4.67, p
= 0.01). Whales 845 and 2083 had longest surface durations during the day, but
these were not different than other time periods (845 KW = 2.12, p = 0.15; 2083
KW = 2.69, p = 0.26).
Duration of all submergences was longer during twilight than during day or night
for whale 837 (F2,456 = 5.38, p = 0.005). Whale 841 had longer submergence
durations during the day than at night or twilight (F2,471 = 15.79, p <0.001). No
significant differences or consistent patterns were seen in submergence durations for
the other whales examined.
Depths of all submergences were shallowest at night for six of the seven whales
examined, three significantly so (F's >4.70, p's <0.01). Daytime dives were
shallowest for the seventh whale.
Significant interactions existed between individuals and time of day for surface
intervals, number of blows per surface, and percentage of time in a surface interval
(F's <2.90, p's <0.04). As such, diel variations in these variables differed
depending on the individual whale (Table 2.6). Whale 837 had longer surface
intervals at night than at other times of day (F2,135 = 6.57, p = 0.002), and more
blows per surface interval at night than during twilight (F2,135 = 4.59, p = 0.01).
Table 2.6: Means ± SE's of visual observation-style variables, by time of day, for all blue whales tagged off southern California,
1994/95. Within each variable for each whale, different superscript letters refer to significant diel differences.
837
WHALE if
TRUE DIVE
Day
DURATION (s)
Twilight
SURFACE
INTERVAL (s)
Day
Twilight
NUMBER
OF BLOWS
Day
Night
Twilight
BLOW
INTERVAL (s)
Day
848
2082
2083
23029
23031
243
306
208
228'
466
40
224
427
± 44
n = 72
± 32
n=3
± 53
n = 11
± 51
n = 54
±6
n=2
± 52
n=6
± 72
377
n = 12
324
53
167
128
184'
±24
±19
±52
±43
±32
±4
±50
n = 46
n = 25
n=8
n = 10
n = 90
n = 10
n = 13
248
345
153
475°
457
370
+ 65
n = 51
+ 26
n = 34
+ 56
+ 48
n = 50
+ 103
25.9'
21.0'
+ 3.8
+ 5.0
Night
845
± 37
n = 41
Night
841
n=3
44.2
58.5
9.9
17.3
67.2
36.9
+ 12.4
+ 16.1
+ 3.4
+ 14.6
+ 19.8
+ 9.2
41.1°
42.3°
19.5
52.7
±5.1
±8.8
±12.2
±14.6
19.1'
26.0'
68.7
±3.4
±4.4
2.3'
±0.3
1.9'
3.3
3.7
±0.2
±0.7
±0.8
2.8'
±0.2
1.8°
±0.2
n=6
±16.1
7.8
±1.7
46.6
3.7
±19.0
1.3
±0.1
1.5
±0.5
3.0°
1.9
2.8
1.2
1.9
±0.6
±0.5
±0.1
±0.4
2.2"
4.0
±1.0
±5.3
±1.2
±0.4
±0.2
26.5
23.9
±8.5
3.3
±0.7
2.7
±0.5
2.3
±0.3
3.2
1.1
±1.1
±0.1
18
18
15
17
26
16
18
+1
n = 21
+1
n = 27
+0
n=3
+2
n=7
+1
+1
+2
n=9
n=8
Table 2.6 (continued): Means ± SE's of visual observation-style variables, by time of day, for all blue whales tagged off southern
California, 1994/95. Within each variable for each whale, different superscript letters refer to significant diel differences.
837
WHALE /
Night
Twilight
% TIME IN
Day
SURFACE
INTERVAL
Night
Twilight
20
Day
Night
Twilight
15
845
17
848
20
2082
2083
23029
23031
16
26
±1
±1
±2
±2
±1
±2
n = 35
n = 17
n=3
n=8
n = 16
n = 10
20
±1
n = 22
n = 22
12.5
±2.6
16.2
±2.0
11.2
52
16
17
±1
±2.1
TRUE DIVE
DEPTH (m)
841
11.8
±2.1
17.1
±4.0
19.4
14.5
±3.8
±8.5
n=3
n=4
18.4
25.2
±6.0
12.8
18.6
±3.2
±3.4
47
±3
±4.0
26
55
21
23
±1
±2
n=5
3.5
24.3
±1.2
±16.8
3.8
±1.0
26.1
±6.9
10.0
23.7
±3.2
±6.9
13.4
1.7
±4.7
±0.7
22`
9.3
±2.7
18
95
63
±12
±7
±4
±21
±5
±2
±34
±24
n = 47
n = 87
n=4
n = 12
n = 63
n=4
n=7
n = 14
20
26
15e
22
13e
23
23
±3
±10
±7
±8
±2
±2
±7
n = 50
n = 27
n=9
n = 11
n = 102
n = 11
n = 14
55
±9
n = 60
49
105
25'
26
±11
±45
±4
±14
n = 37
n=4
n = 56
n=3
52
±35
n=6
64
Whale 841 had longer surface intervals with more blows at night than during the day
(F2,128 = 3.91, p = 0.02; and F2,128 = 4.04, p = 0.02, respectively), but these values
were not different than those during twilight. Significant diel differences were not
seen for the other whales, or for percentage of time in a surface interval.
There was no evidence of an interaction between individual and time of day for
true dive duration, depth of true dives, or mean blow interval (F's <1.60, p's
>0.12). True dives were shorter at night (R = 260 ± 31 sec, n = 184) than during
the day (R = 334 ± 30 sec, n = 190) or at twilight ("R = 360 + 47 sec, n = 144).
Depth of true dives was shallower at night (R = 21 + 5 m, n = 215) than during the
day (R = 43 + 6 m, n = 227) or at twilight (5-( = 52 ± 8 m, n = 166). None of the
above-mentioned differences were significant, however (F's <2.40, p's >0.08).
Mean blow intervals were almost identical for the three times of day (night )7( = 19 ±
1 sec, n = 86; day R = 19 + 1 sec, n = 72; twilight R = 19 + 1 sec, n = 56).
Travel Mode comparisons
Visual inspection of the tracklines of tagged whales revealed two types of
movement for two of the animals (841 and 2083); directional, characterized by longdistance travel in a relatively constant direction, and non-directional, characterized by
shorter-distance travel, with many direction changes. Comparisons of dive habits
during these two types of movement revealed significant differences in many of the
variables for whale 841 (Tables 2.7, 2.8, and 2.9). Percentage of time at the surface
65
Table 2.7: Means ± SE's of summary period variables, by travel mode, for blue
whale 841, tagged off southern California, 1995. Within each variable, different
superscript letters refer to significant differences.
WHALE
841
DIRECTIONAL
TRAVEL
BLOW RATE
(blows/min)
% TIME AT
SURFACE
0.3'
0.8'
±0.2
±0.1
MAX.
SURFACE
DURATION
MAX.
DIVE
DURATION
MAX.
DIVE
DEPTH
(s)
(s)
(in)
1512'
214
3.5'
±0.3
±8
±345
n = 10
NON­
DIRECTIONAL
TRAVEL
4.6°
1.2b
13.5b
439b
241"
+ 0.3
+ 0.1
+ 1.2
+ 24
+2
n = 56
Table 2.8: Means ± SE's of individual dive variables, by travel mode, for blue
whale 841, tagged off southern California, 1995. Within each variable, different
superscript letters refer to significant differences.
WHALE
841
SURFACE
DURATION
DIVE
DURATION
DIVE
DEPTH
(s)
(s)
(m)
DIRECTIONAL
TRAVEL
11 = 45
1.7'
342'
37'
+ 0.2
+ 62
+8
NON­
DIRECTIONAL
TRAVEL
2.7°
59b
+ 0.1
±4
n = 585
16b
+2
66
Table 2.9: Means ± SE's of visual observation-style variables, by travel mode, for
blue whale 841, tagged off southern California, 1995. Within each variable, different
superscript letters refer to significant differences.
WHALE
841
DIRECTIONAL
TRAVEL
SURFACE
INTERVAL
TRUE DIVE
DURATION
(s)
(s)
4.T
±2.1
asr
±100
NUMBER
OF BLOWS
(s)
1.P
±0.1
24
±6
% TIME IN
SURFACE
INTERVAL
TRUE DIVE
DEPTH
2.0
±0.5
(m)
46
±9
n = 2
n = 24
NONDIRECTIONAL
TRAVEL
BLOW
INTERVAL
33.5°
165°
2.6°
17
19.5°
43
± 3.7
+ 15
± 0.2
+1
n = 63
± 2.0
± 5
n = 98
during a summary period was higher during non-directional travel than for directional
travel (F1,64 = 113.19, p <0.001), as was mean blow rate (F1,64 = 120.82, p
<0.001). Non-directional travel was also characterized by longer maximum surface
durations (KW = 19.14, p <0.001), shorter maximum dive durations (F1,64 = 43.79,
p <0.001), and deeper maximum dive depths (F1,64 = 24.72, p <0.001) during
summary periods than directional travel. Duration of individual surfacings was also
longer during non-directional travel than during directional travel (F1,628 = 18.83, p
<0.001), as was surface interval time (F1,120 = 35.12, p <0.001). While engaged in
non-directional travel, whale 841 also exhibited shorter, shallower submergences
(1'1.628 = 89.28, p <0.001; and F1.628 = 24.94, p <0.001, respectively), and shorter
true dives (F1,120 = 11.36, p = 0.001) than while engaged in directional travel.
Depths of true dives did not differ significantly between the two modes of travel,
however (F1,03 = 1.08, p = 0.30). Surface intervals during non-directional travel had
67
more blows (KW = 21.53, p <0.001), and shorter blow intervals than during
directional travel. The latter difference was not significant (KW = 2.16, p = 0.14).
Percentage of time in a surface interval was higher during non-directional travel than
during directional travel (F1,120 = 59.57, p <0.001).
The only variable for which whale 2083 showed a significant difference between
directional and non-directional travel was blow rate, with the rate being higher for
non-directional travel (5-( = 0.5 + 0.0, n = 8) than for directional travel OR = 0.4 +
0.0, n = 2, KW = 4.39, p = 0.04).
Bathymetrical Comparisons
Whale locations were divided into three bottom depth categories: continental shelf
(0-200 m), continental slope (200-2000 m), and deep ocean (2000+ m). Only one of
the eight tagged whales provided information from all three depth categories, so data
were pooled from all whales for analysis. Comparisons of dive habits in the three
bottom depth categories revealed significant differences in many variables (Tables
2.10, 2.11, 2.12). Percentage of time at the surface during summary periods was
higher for whales over the continental shelf waters than for whales in deeper water
(KW = 6.09, p = 0.05). Dive habits over shelf waters were also characterized by
higher blow rates than over the slope or deep ocean (KW = 9.40, p = 0.009) and
higher maximum surface durations during summary periods. The latter difference was
not significant, however (KW = 3.62, p = 0.16). Maximum dive duration during
68
Table 2.10: Means ± SE's of summary period variables, by bottom-depth category, for
all blue whales tagged off southern California, 1994/95. Within each variable, different
superscript letters refer to significant differences.
BLOW RATE
(blows/min)
% TIME AT
SURFACE
SHELF
4.2"
1.1'
(0-200m)
+ 0.3
+0.1
MAX.
SURFACE
DURATION
MAX.
DIVE
DURATION
MAX.
DIVE
DEPTH
(s)
(s)
(m)
5.5
±0.5
376
76
±8
±96
n=2
SLOPE
1.9b
0.6b
5.3
697
164
(200-2000m)
+ 0.1
+ 0.0
+ 0.4
+ 33
+ 10
n = 90
DEEP OCEAN
(2000+m)
1.7.
0.4°
4.3
1619
169
+ 0.6
n=6
+ 0.1
± 1.4
+ 602
± 12
Table 2.11: Means ± SE's of individual dive variables, by bottom-depth category, for
all whales tagged off southern California, 1994/95. Within each variable, different
superscript letters refer to significant differences.
SHELF
(0-200m)
SURFACE
DURATION
DIVE
DURATION
DIVE
DEPTH
(s)
(s)
(m)
3.5'
+ 0.2
n = 600
52'
14'
+4
+1
SLOPE
1.9b
111b
18b
(200-2000m)
+ 0.0
n = 787
±7
+1
DEEP OCEAN
(2000+m)
2.2b
391°
31b
+ 0.2
n = 23
+ 108
+8
69
Table 2.12: Means ± SE's of visual observation-style variables, by bottom-depth
category, for all blue whales tagged off southern California, 1994/95. Within each
variable, different superscript letters refer to significant differences.
SHELF
(0-200m)
SURFACE
INTERVAL
TRUE DIVE
DURATION
(s)
(s)
37.1
±3.7
151
±12
NUMBER
OF BLOWS
BLOW
INTERVAL
(s)
2.7°
17'
% TIME IN
SURFACE
INTERVAL
TRUE DIVE
DEPTH
20.4
(m)
40
±5
+0.2
±1
±1.8
20'
+1
11.2b
37
+ 1.0
+4
n = 114
SLOPE
(200-2000m)
DEEP
OCEAN
(2000+ m)
26.0'.
281b
2.2°
+ 2.1
n = 226
+ 17
+ 0.1
3.7°
438°
1.1°
4Ab
+1.8
+142
+0.1
+2.4
32
+9
n = 17
summary periods was shortest, although not significantly so, over shelf waters (KW =
4.56, p = 0.10). Maximum dive depth during summary periods was shallowest over
shelf waters. This difference was also not significant (KW = 1.06, p = 0.59).
Individual surface durations were also longer for whales over continental shelf
waters than for whales over the slope or deep ocean (F
. 2.1407 = 69.84, p <0.001).
Individual dives over the shelf were of shorter duration, and to shallower depths than
dives over the slope or deep ocean (F2,1407 = 46.69, p < 0.001; and F2,1407 = 3.64, p
= 0.03, respectively).
Duration of surface intervals was longer for whales over shelf waters than for those
over the slope or deep ocean (F2,354 = 14.50, p <0.001). Duration of true dives was
70
shorter over shelf waters than over the slope or deep ocean (F2,354 = 8.46, p <0.001).
Depth of true dives did not differ between the three depth categories (F2,457 = 0.23, p
= 0.79). While over shelf waters, whales blew more times per surface interval than
when over the slope or deep ocean (KW = 22.20, p <0.001), and had shorter mean
blow intervals (F
.- 1,191 = 8.04, p = 0.005). Only one blow interval was recorded for
whales over the deep ocean, and could not be included in the analysis. Percentage of
time spent in a surface interval was highest over shelf waters followed by slope waters
and deep ocean (F2,354 = 24.44, p <0.001).
Within each bottom depth category, significant diel differences also existed in some
of the dive variables. Summary variables for shelf waters were not included in these
comparisons because of inadequate sample sizes.
While over shelf waters, whales had longer surface durations during the day than
during either twilight or night (F
.- 2,482 = 8.22, p <0.001). Depths of all submergences
were shallower at night than during other times of day (F2,482 = 35.54, p <0.001).
None of the other variables exhibited significant diel differences over shelf waters.
Whales over slope waters had significantly shorter true dives at night and twilight
than during the day (F2,223 = 4.47, p = 0.01). Maximum dive durations during
summary periods were also shorter at night than at other times of day (F2,87 = 9.40, p
<0.001). Percentage of time in a surface interval was longer during night and twilight
than during the day (F2,223 = 4.34, p = 0.01). Maximum surface durations during
summary periods were also longer at night than at other times of day (F2,87 = 5.76, p
71
= 0.005). Maximum dive depths, depths of all submergences, and depths of true dives
were all shallower at night than at other times of day (KW = 34.87, p <0.001; F2,724
= 7.07, p = 0.001; and F2,268 = 3.30, p = 0.04, respectively).
Whales over the deep ocean did not exhibit significant diel differences in any of
the dive variables.
DISCUSSION
This study provides remotely-telemetered respiration and dive information from
eight free-ranging blue whales over periods of 1-39 days. It is unique in that it
combines tag data and visual observation data for ground-truthing and comparison, and
allows for more educated inferences on the behavior of tagged whales.
Other than Lagerquist et al. (in prep), there are no current respiration and dive data
available for blue whales with which to make comparisons. Much of the data obtained
in this study are in close agreement with values obtained from tagged blue whales in
1993. Mean duration of true dives in this study was 5.3 min, and in 1993, average
duration of dives greater than one minute (true dives) was 5.9 min. Mean duration of
all submergences was 2.1 min in this study and 1.8 min in 1993. Mean maximum dive
duration from this study was 11.3 min compared to 13 min for one whale in 1993.
Percentage of time at the surface in this study was 2.7 % compared to 4.4 % in 1993.
Blow rate in this study was 0.7 blows/min compared to 0.6 blows/min in 1993. Dive
depths were not so similar, however. Mean depth of all submergences in this study was
72
18 m compared to 30 m in 1993. Mean depth of true dives was 35 m compared to a
1993 value of 102 m. The large difference in true dive depth between the two studies
could be explained by the fact that in 1993 true dives were defined as anything over 1
min in duration, due to the type of data collected. We did not have detailed resolution
in duration of dives less than one minute in length. In the current study, true dives were
defined as any submergence greater than 32 sec. Thus shorter, and as a result
shallower, dives were included in the mean for this study. Mean maximum depth of
dives during summary periods was 177 m in this study, compared to 151 m in 1993.
Unlike tagged blue whales in 1993, significant correlation existed between durations
and depths of all submergences. Lack of such a correlation in 1993 could have been
attributed to small sample sizes. Durations and depths of true dives were also positively
correlated, although not as much as for all submergences. Removal of blow intervals
from correlation analysis may have been the cause for the lower correlation for true
dives. Dolphin (1987) found similar correlations between dive time and depth of dives
for foraging humpbacks in Alaska.
Data from tagged whales and untagged whales in this study were not so similar,
however. Mean duration of individual surfacings was 2.6 sec for tagged whales,
compared to 6.4 sec for untagged whales. Method of data collection could explain this
difference. The value for tagged whales represents the amount of time for which the tag
was dry, whereas the value from visual observation of untagged whales represents the
time for which any part of the whales' body was visible above the waters' surface, and
73
would be longer. The mean duration of all submergences was 128 sec for tagged
whales compared to 40.2 sec for untagged whales. Again, data collection methods
could explain this difference. More data are available from tagged whales, and
therefore more opportunity for longer dives to be represented. Additionally, visual
observation studies bias dive duration downward, due to the difficulty of resighting a
focal animal after a long dive (Dorsey et al. 1989). Mean duration of surface intervals
was longer for untagged whales than for tagged whales. The number of blows per
surface interval for tagged whales was much less than for untagged whales, and this
brought the mean surface interval for tagged whales down. Mean duration of true dives
was longer for tagged whales than untagged whales, but was expected due to the reason
mentioned above. Mean blow interval was also longer for tagged whales than untagged.
The method of data collection affected this, as the tags did not register submergences
shorter than 6 sec, whereas all submergences were included in the analysis for untagged
whales. Additionally, log-survivorship analysis resulted in a cutoff of 24 sec to be used
in the distinction between blow intervals and true dives from visual observations.
Similar analysis for tag data resulted in a 32 sec cutoff. Therefore, longer
submergences were included in the calculation of mean blow interval for tagged
animals.
Dive durations and surface intervals from visually-observed whales were correlated
in this study. The fact that surface intervals were more closely related to the duration of
the previous dive than to the duration of the following dive suggests a period of
74
recovery rather than preparation for an upcoming dive. Correlations between surface
intervals and durations of previous dives were also found for three of the tagged
whales. This finding is consistent with that of Dorsey et al. (1989) for summering
bowhead whales.
As in Lagerquist et al. (in prep.), dive habits of individual whales differed
significantly. After controlling for these differences, diel differences also occurred.
Data from all whales combined showed the animals spending longer periods of time at
the surface at night than during the day or at twilight. This was apparent in percentage
of time at the surface and maximum surface duration during summary periods.
Percentage of time at the surface was not significantly different between the different
times of day, however. When examined individually, five of seven whales also
reported longest surface durations at night, but only three of these were significantly
longer than other time periods. Durations of surface intervals were longest at night for
three whales (only two significantly so), and during twilight for two other whales
(neither significantly so). Whales dived for shorter periods of time at night than during
the day or twilight, as evidenced by maximum dive duration during summary periods.
True dive duration was also shorter at night, but not significantly so. Depths of true
dives were shallower at night (although not significantly so) than during the day or
twilight for all whales as a group. Six of seven whales examined individually had
shallower maximum dive depths at night than at other times of day (two significantly
so). Depths of all submergences were also shallowest at night for six of seven whales
75
(three significantly so). The diel differences in dive durations and depths noted here are
consistent with the dive pattern of a single blue whale tagged in 1993 (Lagerquist et al.
in prep.). No other comparable data exist for blue whales. Radio-tracked fin and
humpback whales in Alaska (Watkins et al. 1981) and a fin whale in Iceland (Watkins
et al. 1984), also exhibited shorter dives at night than during the day. Watkins et al.
(1984) attributed the longer daytime dives of the fin whale in Iceland to subsurface
feeding. Right whales in the Great South Channel region east of Cape Cod engaged in
short surfacings, long dives during the day, and shorter dives at night during a year in
which their zooplankton prey underwent strong diel vertical migration (Winn et al.
1995).
Other respiration studies have been conducted visually rather than by telemetry, and
therefore do not provide data at night. They do provide correlations between dive
habits and behaviors that may shed some light on the results obtained in this study.
Kopelman and Sadove (1995) found dive durations to be shorter for surface-feeding fin
whales than for non-surface-feeding whales off Long Island, New York. Surfacefeeding bowhead whales also had longer surface times and blow intervals, and shorter
dives than bottom-feeding bowheads, or bowheads engaged in other activities (Dorsey
et al. 1989). Visually-observed blue whales in this study exhibited differences in dive
habits among specific behaviors, with feeding blue whales having shorter dives than
whales engaged in other behaviors.
76
Blue whales have recently been congregating each summer and fall near the
Channel Islands to feed (Barlow et al. 1995), and it seems possible that the night-time
dive behavior of the tagged animals in this study could represent surface-feeding on
vertically migrating krill (such as Euphausia pacifica). Two whales (837 and 2082)
exhibited longest, deepest dives during twilight, which could reflect pursuit of prey
ascending and descending in the water column at sunset and sunrise. This explanation
was also suggested to explain the long morning and evening dives of a radio-tracked fin
whale in Iceland (Watkins et al. 1984).
Whale 841 exhibited shorter, shallower dives, with more time at the surface during
non-directional travel. This is similar to the pattern exhibited by feeding whales, or
tagged whales at night. Animals engaged in non-directional travel would perhaps have
more time to spend feeding. The mean maximum dive depth during summary periods
was deeper during non-directional travel, which seems somewhat contradictory to the
rest of the dive pattern for this travel mode. Perhaps whales were engaging primarily in
shallower shorter feeding dives during non-directional travel, with the occasional deep
dive to sample the water column. Winn et al. (1995) reported right whales east of Cape
Cod occasionally diving deeper than their main prey patch, possibly as a means of
detecting changes in depth distribution of prey patches. In contrast, during directional
travel, dives were more consistently deeper. During directional travel, whales may
swim at deeper depths to avoid the turbulent surface waters, thus conferring
hydrodynamic advantage to their movements. Traveling whales observed visually had
77
dive patterns similar to tagged whales engaged in directional travel: longer than average
dives, lower than average blow rates, longer blow intervals, and lower than average
percentage of time in a surface interval. Bowhead whales have also been shown to have
longer blow intervals, longer dive durations, and lower blow rates during migration
than when on their summer feeding grounds (Wursig et al. 1984). Lower blow rates for
traveling animals have also been shown for gray (Wursig et al. 1986) and humpback
whales (Dolphin 1987).
Dive habits of whales over shelf waters have a similar pattern to feeding whales,
whales engaged in non-directed travel, and whales at night. Dives over shelf waters
were typically of shorter duration, and to shallower depths than dives over deeper
water. Whales over shelf waters also spent more time at the surface, had shorter blow
intervals, and higher blow rates than when over deeper water. Percentage of time spent
in a surface interval was also highest over shallower water. Bowhead whale dives have
been shown to increase with increasing water depth, but it was unclear whether this was
a water depth, or year affect (Dorsey et al. 1989). It is also unclear for this study
whether this change in pattern with bottom depth is real or perhaps just an artifact of a
change from non-directed, feeding behavior over shallow water, to more directed,
migratory behavior as the whales leave the area and head out over deeper water.
Upon closer examination of dive habits within each bottom depth category, we
found that diel differences were exhibited over slope waters more so than over the
shelf. At nighttime over the slope, whales were engaged in short, shallow dives, with
78
long surface durations, typical of feeding behavior. The occurrence of a diel diving
strategy over slope waters and not over the shelf is not unexpected in light of blue
whale prey habits. The euphausiid, Thysanoessa spinifera, commonly consumed by
blue whales in California (Schoenherr 1991, Lagerquist et al. in prep.), is a non­
vertically-migrating species found only over shelf waters (Boden 1955, Brinton 1962).
Euphausia pacifica, another commonly consumed euphausiid species, occurs further
offshore, and engages in vertical migration, rising toward the surface at night (Brinton
1962). Blue whales over slope waters may feed primarily at night when prey is more
accessible.
This study has documented the change in many diving and respiratory
characteristics of blue whales depending on time of day, behavior (including travel
mode), and water depth. Most importantly for sighting correction, percentage of time
spent at the surface appears to be affected by many of these factors. Thus, such
variables need to be considered when choosing correction factors for abundance
estimation.
79
SUMMARY
These are the first studies to track blue whales by satellite telemetry. They
demonstrate the effectiveness of using this technique to monitor the movements and
dive habits of this exceedingly large, free-ranging, pelagic cetacean. Satellite tracking
may in fact be the only way to monitor such behaviors of blue whales over prolonged
periods of time, both day and night, in all weather conditions, and over large
geographic areas, with minimal stress to the animal.
The first study, off central California, provides preliminary evidence of individual
variation, as well as diel differences, in blue whale dive habits. One whale exhibited
more frequent, shorter duration dives during the day than at night, while two others
exhibited the opposite pattern. One of the latter two whales also had shallower dives at
night than during the day. The fourth whale in the study dove more frequently, but for
longer duration during the day than at night. Such differences in diving strategies may
reflect individual foraging habits, or differences in prey species.
The second study, off southern California, provides additional evidence of
individual variation in many of the dive parameters, as well as diel differences, and
differences in dive habits depending upon water depth and behavior. The majority of
the tagged whales in this study exhibited longer surface durations and shorter,
shallower dives at night than during the day or twilight. Shorter dive durations were
also characteristic of feeding, untagged, whales. Dives over shelf waters (0-200 m)
were typically of shorter duration and shallower depth, with more time spent at the
surface, than dives over deeper water. Significant diel differences in dive habits were
exhibited by whales over slope waters (200-2000 m), with dives being shorter and
80
shallower, with longer surface durations, at night than during the day or twilight. One
whale exhibited short, shallow dives, with more time at the surface, while engaged in
non-directional travel. During directional travel, this whale exhibited fewer, longer,
deeper dives, with less time at the surface, similar to the pattern exhibited by untagged,
traveling whales. Depth and duration of dive were positively correlated for the tagged
whales in this study. Positive correlations were also found between duration of surface
intervals and dives for both tagged and untagged whales. In the case of all untagged
whales, and two tagged whales, duration of surface interval was more closely related to
the duration of the previous dive than to that of the subsequent dive.
In both studies, differences in average values for many of the dive parameters
existed between tagged and untagged whales. The majority of these differences may be
attributable to the method of data collection. This emphasizes the need to view
comparisons to other dive habit studies with caution, taking into account the inherent
biases associated with different data collection techniques.
The nature of the two studies outlined here dictate the conclusions one can draw
from them. Both were observational studies, meaning inferences cannot be made to the
species, or even the population, as a whole. Neither the visually-observed nor the
tagged whales in these studies represent random samples from any larger population.
Behavioral or physical characteristics of these particular whales may have been such as
to enable us to closely approach them for observation and tagging. As such, statistical
inferences may be made only to the individual whales in these studies. But this does not
render the results useless or uninteresting. The results provide detailed information
concerning the diving capabilities of the blue whales in these studies. Such specifics can
help in the software and hardware design of new tag technology. The results also give
81
scientists baseline data upon which to develop additional questions and base further
study.
One of the goals of future tagging studies would be to develop smaller, lighter tags
in the hopes of achieving longer periods of attachment to the animals, thereby
providing more data. Future studies should include the concurrent monitoring of prey
abundance and distribution, as well as physical oceanographic information, such as sea
surface temperature, current information, or weather conditions. Such information
would help uncover the significance of different diving strategies, and provide
additional insight into the behavior of free-ranging blue whales.
82
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